chore(core): the internal module for size-change glides (spec:AST-105, INV12)

.changeset/animated-layout.md

@@ -0,0 +1,7 @@
+---
+'@astryxdesign/core': patch
+---
+
+[chore] Internal: the one module for size-change glides (`spec:AST-105`, `architecture:react-component-runtime` INV12). After a reported size change, the content after it inside a container glides to its new place on one shared internal motion role, and the changed element enters or exits on the same motion. Not exported; no component uses it yet. The public `useAnimatedLayout` hook and Collapsible's and TreeList's `animatedLayout` prop arrive together in a later release.
+
+@cixzhang

docs/architecture/react-component-runtime.md

@@ -164,8 +164,9 @@ those contracts.
   `spec:AST-105` defines. It measures the layout before and after a reported
   change and moves shifted content with a compositor-only translate played
   through the shared rectangle morph (`architecture:layer-runtime` INV15;
-  `spec:AST-094` FR5); it never animates height. Its walk visits only the
-  following element siblings at each ancestor level up to the area, through
+  `spec:AST-094` FR5) once Core has that morph; until then, the module plays
+  its own compositor translate. It never animates height. Its walk visits only
+  the following element siblings at each ancestor level up to the area, through
   `parentElement` and `nextElementSibling`, descending only into an element
   with `display: contents`, so its cost grows with depth plus following
   siblings. Changes in one area in the same frame are measured together. A

docs/specs/AST-105-animated-layout/spec.md

@@ -126,6 +126,10 @@ Out of coverage, each taking its final place at once:
   since only content after the change in the DOM moves;
 - text reflowing inside a paragraph beside a float;
 - elements the app moves itself with its own transforms;
+- a closing element whose space no end margin frees (a grid item whose
+  track or gap changes when it hides, or a flex-row item that sets its row's
+  height): it hides at once, without its exit, and the content after it
+  takes its final place at once;
 - an anchored popover tied to a moving element, which layer anchoring keeps
   in place (`architecture:layer-runtime`).