Model v2 anchor

docs/wg/feat-paragraph/index.md

@@ -1,14 +1,33 @@
 ---
-title: Paragraph Feature Roadmap
+title: Paragraph and Text Layout
+description: "Specifications, focused studies, and implementation coverage for paragraph and typography behavior."
 tags:
   - internal
   - wg
   - typography
   - text
   - fonts
+format: md
 ---
 
-# Paragraph Feature Roadmap
+# Paragraph and Text Layout
+
+## Specifications
+
+- [Universal Shaped Text Layout](./text-layout) defines the canonical
+  resolution contract shared by geometry-sensitive consumers.
+
+## Focused studies
+
+- [Font fallback](./impl-font-fallback)
+- [Italic and oblique](./impl-italic)
+- [Optical size automation](./impl-opsz-auto)
+- [Emoji placeholder rendering](./impl-emoji-placeholder)
+
+## Coverage inventory
+
+The checklist below records implementation coverage. It does not define
+measurement, shaping, line, glyph, or editing geometry semantics.
 
 **Core / Modeling**
 
@@ -65,7 +84,7 @@ tags:
   - [x] text-decoration-thickness (only supports %)
   - [ ] text-decoration-skip-ink - https://github.com/rust-skia/rust-skia/issues/1187
   - [ ] known limitations https://github.com/gridaco/grida/issues/416
-- [x] [Variable axes](https://github.com/gridaco/grida/blob/canary/docs/reference/open-type-variable-axes.md)
+- [x] [Variable axes](../../reference/open-type-variable-axes)
   - [x] `wght`
   - [x] `wdth`
   - [x] `slnt`
@@ -84,7 +103,7 @@ tags:
   - [x] `YTAS`
   - [x] `YTDE`
   - [x] `YTFI`
-- [x] [open type featuers](https://github.com/gridaco/grida/blob/canary/docs/reference/open-type-features.md)
+- [x] [OpenType features](../../reference/open-type-features)
 
 **Text Editor**
 

docs/wg/feat-svg/_category_.json

@@ -1,3 +1,7 @@
 {
-  "label": "SVG"
+  "label": "SVG",
+  "link": {
+    "type": "doc",
+    "id": "wg/feat-svg/index"
+  }
 }

docs/wg/feat-svg/animation-composition.md

@@ -0,0 +1,283 @@
+---
+title: "SVG Animation Profile 3: Additive Composition"
+description: "A deterministic SVG animation subset for typed additive sandwiches and cumulative repeat iterations."
+keywords:
+  - svg
+  - animation
+  - smil
+  - animation sandwich
+  - additive animation
+  - cumulative animation
+  - deterministic rendering
+tags:
+  - internal
+  - wg
+  - canvas
+  - svg
+  - rendering
+format: md
+---
+
+# SVG Animation Profile 3: Additive Composition
+
+**Status:** Accepted source profile.
+
+SVG Animation Profile 3 extends [SVG Animation Profile
+2](./animation-sandwiches) with typed addition between sandwich layers and
+terminal-value accumulation across repeat iterations. It is cumulative: every
+source accepted by Profile 2 is accepted by Profile 3 and produces the same
+sampled values, including the same binary32 bits.
+
+All inherited source, timing, interpolation, priority, target, failure, and
+trust-boundary rules remain normative unless this document replaces them.
+Profile 3 replaces only the composition grammar and the replacement-only
+sampling procedure. It does not add `by`, lone-`to` animation, another target
+element, or another animated property.
+
+The behavior follows SMIL's [additive and cumulative animation
+model](https://www.w3.org/TR/2001/REC-smil-animation-20010904/#AnimFuncAdditive)
+inside the deliberately bounded source and numeric domains of Profiles 0–2.
+
+## Composition vocabulary
+
+Profile 3 admits these values on every otherwise admitted `<animate>`:
+
+| Attribute    | Absent means | Admitted values  | Operation                                     |
+| ------------ | ------------ | ---------------- | --------------------------------------------- |
+| `additive`   | `replace`    | `replace`, `sum` | composition against the lower sandwich result |
+| `accumulate` | `none`       | `none`, `sum`    | composition across repeat iterations          |
+
+The attributes are orthogonal. All four combinations are admitted for `x`,
+`y`, `width`, `height`, and `opacity`. Unknown values are errors; a processor
+must not treat them as the absent defaults.
+
+Effect composition and iteration composition have different operands:
+
+- `additive="replace"` makes the effect replace its lower-priority underlying
+  sandwich value.
+- `additive="sum"` adds the effect to that underlying value.
+- `accumulate="none"` uses the effect's simple value for the current
+  iteration.
+- `accumulate="sum"` first adds a multiple of the effect's terminal value to
+  its current simple value. The resulting effect is then replaced or added as
+  selected by `additive`.
+
+The operations are typed property operations, not string concatenation and
+not a second untyped value channel. Each result projects back into the same
+property representation as its target.
+
+## Iteration accumulation
+
+Let:
+
+- `V(p)` be the inherited simple animation value at progress `p`, after
+  keyframe selection, easing, and interpolation;
+- `T` be the terminal simple value at the end of one simple duration; and
+- `k` be the zero-based repeat index.
+
+For `from`/`to`, `T` is `to`. For `values`, `T` is the last list item,
+regardless of which keyframe interval contains `p`. For a one-value animation,
+that one value is both `V` and `T`.
+
+The effect value is:
+
+```text
+E(p, k) = V(p)                         when accumulate = none
+E(p, k) = V(p) + k × T                 when accumulate = sum
+```
+
+Accumulation uses the terminal value itself, not the change `T - V(0)` and
+not the value sampled immediately before a repeat boundary. When `k = 0`, both
+accumulation modes are bit-identical.
+
+For an underlying sandwich value `U`, the four combinations are:
+
+| `additive` | `accumulate` | Result for this layer | Cuts off lower layers |
+| ---------- | ------------ | --------------------- | --------------------- |
+| `replace`  | `none`       | `V(p)`                | yes                   |
+| `sum`      | `none`       | `U + V(p)`            | no                    |
+| `replace`  | `sum`        | `V(p) + k × T`        | yes                   |
+| `sum`      | `sum`        | `U + (V(p) + k × T)`  | no                    |
+
+Iteration accumulation never changes sandwich priority. In particular,
+`additive="replace" accumulate="sum"` is still a replacement layer and is a
+cutoff; accumulation changes its value, not its relationship to lower layers.
+
+## Repeat and fill boundaries
+
+Profile 0's begin-inclusive, end-exclusive active interval remains in force.
+At an exact internal repeat boundary, the next iteration has begun: `k`
+increments by one and `p` is exactly zero. An accumulated effect therefore
+contributes `V(0) + k × T` at that instant. Sampling just before the boundary
+still uses the preceding iteration and its progress.
+
+At the final active end:
+
+- `fill="remove"` contributes nothing and immediately falls through to the
+  next contributing layer or the authored base; and
+- `fill="freeze"` uses terminal progress with `k = N - 1`, where `N` is
+  `repeatCount`.
+
+Consequently a frozen `accumulate="sum"` effect has value `N × T`. A frozen
+additive effect continues to add that value at its unchanged sandwich
+priority. Direct seeking to the final end and arriving there through playback
+must agree.
+
+## Additive sandwich procedure
+
+For each target/property pair, contributing effects are first ordered from
+lowest to highest priority by Profile 2. Active and frozen effects contribute;
+pre-begin and post-remove effects do not.
+
+The result is then formed as follows:
+
+1. Find the highest-priority contributing effect whose `additive` operation is
+   `replace`.
+2. Discard every contributing effect below that replacement. If a replacement
+   exists, its accumulated effect value is the initial result. Otherwise the
+   authored base value is the initial result.
+3. Visit each remaining higher-priority effect in ascending priority. Compute
+   its iteration-accumulated effect value, then replace or add it according to
+   `additive`.
+4. Project the final typed value back to the target property and emit at most
+   one effective value for that target/property pair.
+
+This is an ordered fold, not an algebraic reduction. Additions must not be
+reassociated, regrouped, or evaluated in document order after priority has
+been resolved. A replacement masks both the authored base and all lower
+effects, while higher additive effects still compose over it.
+
+Every layer and live target is validated before sampling, including inactive
+and masked layers. Arithmetic below the highest contributing replacement
+cutoff is not evaluated, because it cannot influence the result. Thus a stale
+masked target is still an error, but numeric overflow in a masked lower
+effect's unused composition is not.
+
+## Typed domains and authored base
+
+The source domains of Profile 0 remain unchanged:
+
+| Property          | Admitted authored/keyframe values    | Additive underlying value |
+| ----------------- | ------------------------------------ | ------------------------- |
+| `x`, `y`          | finite binary32 SVG number           | start-edge coordinate     |
+| `width`, `height` | finite, non-negative binary32 number | fixed extent              |
+| `opacity`         | binary32 number in `[0, 1]`          | scalar opacity            |
+
+An animation sandwich containing any `additive="sum"` layer requires the
+authored base to retain that exact numeric projection. An `x` or `y` base
+represented by a center pin, end pin, or two-edge span is not silently
+converted to a start-edge coordinate. An automatic size is not silently
+resolved into a fixed extent. The requirement is checked for the compiled
+source snapshot and again when sampled, so a later structural change cannot
+alter the meaning of addition.
+
+The requirement applies even when the additive layer is inactive or happens
+to sit below a contributing replacement at one requested time. This keeps
+program validity independent from contribution state and prevents a masked
+source error from appearing only after a seek.
+
+A replacement-only sandwich does not need the base as an arithmetic operand,
+but it still inherits Profile 0's target-identity and one-to-one
+materialization requirements.
+
+## Exact numeric contract
+
+Profile 1's exact keyframe interpolation produces one stored binary32 simple
+value. Profile 3 adds two separately rounded operations:
+
+1. **Accumulation:** interpret the stored `V` and `T` bits as exact values,
+   evaluate `V + k × T` mathematically, then round once to finite IEEE 754
+   binary32 using round-to-nearest, ties-to-even.
+2. **Sandwich addition:** interpret the current underlying bits and the
+   already accumulated effect bits as exact values, add them mathematically,
+   then round once by the same rule.
+
+Every additive layer performs step 2 independently in low-to-high priority
+order. Accumulation and sandwich addition are never fused into one rounding,
+and several sandwich additions are never collapsed into a total. Replacement
+copies the accumulated effect bits without another arithmetic rounding.
+
+An exact zero sum is negative zero only when both addends are negative zero;
+otherwise it is positive zero. Any accumulation or influential sandwich
+addition whose rounded result is non-finite is an error rather than a
+saturated value.
+
+`opacity` is the one final projection clamp. Its composed binary32 values are
+not clamped between layers. After the complete sandwich has been folded, a
+value below zero projects to zero, a value above one projects to one, and a
+value already in `[0, 1]` is unchanged. This does not relax the inherited
+`[0, 1]` source-keyframe domain. Coordinates and extents have no corresponding
+final clamp.
+
+## Validation and diagnostics
+
+Profile 3 inherits whole-document atomic validation. In addition to inherited
+failures, Sample rejects:
+
+- an `additive` value other than `replace` or `sum`;
+- an `accumulate` value other than `none` or `sum`;
+- an additive sandwich whose authored base lacks the required typed numeric
+  projection or violates that property's source domain, at compilation or
+  after a later document change; or
+- a non-finite result from influential accumulation or sandwich addition.
+
+Diagnostics identify the source-located animation, target/property pair, and
+failed operation. An arithmetic failure identifies the failing effect and the
+already-applied prefix on which it depends; later effects that were never
+evaluated are not implicated. An unsupported authored-base shape names both
+the required numeric projection and the observed shape, while an invalid
+scalar names the violated domain. A masked or inactive stale target continues
+to identify all layers in its target sandwich. Failure produces neither a
+partial effective-value set nor a partial frame.
+
+## Conformance
+
+Profile 3 adds these laws to the inherited conformance contract:
+
+- every Profile 2 source samples bit-identically under Profile 3;
+- all four `additive`/`accumulate` combinations are tested at the first
+  keyframe, an interior sample, every repeat boundary, and the final end;
+- `accumulate="sum"` uses the terminal keyframe value, including for a
+  non-monotonic `values` list whose terminal differs from an earlier maximum;
+- at an internal boundary, the result is the next iteration's first value plus
+  its terminal-value accumulation;
+- the frozen cumulative effect value is `repeatCount × T`, while remove falls
+  through at the same exact end time;
+- `replace` remains the sandwich cutoff when paired with `accumulate="sum"`;
+- additions above the cutoff run in low-to-high priority order with one
+  binary32 rounding per operation;
+- the authored base is used only when no contributing replacement cuts it off;
+- an additive stack rejects unsupported authored-base projections even when
+  the additive layer is currently inactive or masked;
+- non-finite influential arithmetic rejects Sample, while unused arithmetic
+  below a replacement cutoff is not evaluated;
+- `opacity` is clamped once after the final composition rather than after each
+  layer; and
+- every accepted direct seek is independent of prior sample order.
+
+A canonical scalar matrix uses authored base `10`, `from="20"`, `to="30"`,
+`dur="1s"`, `repeatCount="3"`, and `fill="freeze"`. At times `0`, `0.5`, `1`,
+`1.5`, `2`, `2.5`, and `3` seconds, the required results are:
+
+| `additive` | `accumulate` | Required sequence             |
+| ---------- | ------------ | ----------------------------- |
+| `replace`  | `none`       | `20, 25, 20, 25, 20, 25, 30`  |
+| `sum`      | `none`       | `30, 35, 30, 35, 30, 35, 40`  |
+| `replace`  | `sum`        | `20, 25, 50, 55, 80, 85, 90`  |
+| `sum`      | `sum`        | `30, 35, 60, 65, 90, 95, 100` |
+
+These values are semantic vectors. Bit-level conformance additionally uses
+non-associative boundary operands that distinguish ordered per-layer rounding
+from regrouped arithmetic. Browser differential results remain separate from
+the profile's exact oracle because browser arithmetic and the wider SVG source
+domain are not this profile's numeric contract.
+
+## Deferred families
+
+Profile 3 resolves addition and terminal-value repeat accumulation only for
+the inherited scalar property set. `by`, lone-`to` animation,
+`calcMode="discrete"`, `calcMode="paced"`, transforms, motion paths, colors,
+paint servers, lists and paths, additional SVG targets and properties, wider
+presentation-clamped source domains, begin lists, timing dependencies,
+restarts, indefinite timing, CSS/Web Animations interaction, and optimized or
+compositor execution remain outside this profile.