docs: point at the hosted endpoint, and at what it cannot do

README.md

@@ -155,6 +155,49 @@ blazon dist -r truchet
 Colour is derived in OKLCH with gamut mapping, held in a lightness band that
 keeps contrast on both a white page and a dark terminal.
 
+## Hosted marks
+
+If all you want is a mark in a README, there is an endpoint for it:
+
+```
+https://api.blazon.build/v1/mark/1.4.2.svg
+https://api.blazon.build/v1/mark/1.4.2.svg?renderer=flowfield&bg=dark
+https://api.blazon.build/v1/mark/1.4.2.png?renderer=bishop&size=112
+```
+
+`/v1/mark/{version}.{svg|png|txt}`, with `renderer`, `size`, `padding`,
+`policy`, `palette`, `bg`, `cols` and `nocolor` spelled exactly as the CLI
+spells them. `/v1/renderers` lists what exists and the current size ceilings;
+`/v1/healthz` reports which release is deployed. A mark is a pure function of
+its URL, so the answers are cacheable forever and are served that way.
+
+It runs the same code this repository publishes — the deploy compares every
+response against a `go run` of the same version, byte for byte, and refuses to
+ship on a disagreement. So the marks are the marks. What differs is the budget.
+
+**Prefer SVG.** It is the format the service is comfortable with: its cost is
+whatever the renderer's own maths costs and does not grow with the requested
+size, so it is served at any size up to 1024 with no per-renderer limit.
+
+**PNG is capped**, because rasterising and deflating cost the square of the
+edge length and the deployment has ten milliseconds of CPU per request. The
+ceilings are measured, published at `/v1/renderers`, and currently run from 48
+to 112 pixels depending on the renderer. `flowfield` is not served as PNG at
+all: its streamline integration costs the same at sixteen pixels as at
+five hundred, so there is no size at which it fits.
+
+These are limits of one small deployment, not of the library. `blazon.PNG` has
+no ceiling, no renderer is excluded, and rendering locally costs a few
+milliseconds:
+
+```go
+data, err := blazon.PNG("1.4.2", blazon.Options{Renderer: "flowfield", Size: 1024})
+```
+
+The service is best effort and carries no availability promise. Anything that
+matters should render its own marks — that is the whole point of a library
+with no dependencies.
+
 ## Gallery
 
 Generated by `blazon gallery` and verified in CI: if the committed images stop