…#510)

## Summary

Ships the Canvas 2D half of Tier-1 #8 of #446. This is the surface
`idaptik-ums` (App.res: 1178 LoC of DOM + canvas) and every non-Pixi UI
in the estate has been waiting for; WebGL / WebGL2 / WebGPU stays at `○`
until a consumer surfaces a concrete need.

## What lands

`stdlib/Canvas.affine` (+170 lines, new module): 1 extern type (`Ctx2D`)
+ 26 extern fns covering the full idaptik-ums-relevant Canvas 2D
surface:

| Surface | Externs |
|---|---|
| Context acquisition | `canvasGetContext2D` |
| Styles | `canvasFillStyle` / `StrokeStyle` / `LineWidth` /
`GlobalAlpha` |
| Rectangles | `canvasFillRect` / `StrokeRect` / `ClearRect` |
| Paths | `canvasBeginPath` / `ClosePath` / `MoveTo` / `LineTo` / `Arc`
/ `Fill` / `Stroke` |
| Transform stack | `canvasSave` / `Restore` / `Translate` / `Rotate` /
`Scale` |
| Text | `canvasFont` / `TextAlign` / `TextBaseline` / `FillText` /
`StrokeText` / `MeasureText` |
| Images | `canvasDrawImage` / `canvasDrawImageScaled` |

`lib/codegen_deno.ml` (+58 lines): 26 `__as_canvas*` prelude helpers +
26 dispatch entries adjacent to the Ipc block.

`tests/codegen-deno/canvas_smoke.{affine,harness.mjs}` (+160 lines
combined): 5 distinct smoke functions exercise every shipped extern. The
`MockCtx2D` records every method call as a typed-op tuple so the test
asserts call order + arguments, not just side-effects. The
`canvasMeasureText` round-trip exercises the `Json`-return shape (the
`TextMetrics`-shaped object).

`docs/bindings-roadmap.adoc` row #8 status promoted `○ → ◑`; deferred
items captured (WebGL, gradients/patterns, ImageData, curve primitives,
compositing/clip).

## Design notes

**Why is `HTMLCanvasElement` not its own extern type?** The
canvas-creation entry point is host-dependent — browser
`document.createElement("canvas")` vs idaptik's pre-existing DOM tree vs
jsdom-under-Deno. Treating the canvas as opaque `Json` defers the typed
wrapper to the natural follow-up once `affinescript-dom` lands runtime
support (currently blocked on the wasm-codegen `for-in` / `while` gap,
issue #255).

**Why fix `arc` counter-clockwise to `false`?** The 6-arg shape (`ccw`
boolean) would force every call site to pass a literal `false`. The
5-arg shape covers the overwhelming majority of consumers; a typed
wrapper with the `ccw` flag is a follow-up if a consumer surfaces the
need.

**Why two separate `drawImage` externs instead of one with optional
args?** AffineScript doesn't have JS-style variadic functions; making
`w` and `h` `Option<Float>` would force every natural-size call to
thread `None`s. The two-extern split keeps each call site simple.

**Why open-string `eventMode` / `textAlign` / `textBaseline`?** Same
rationale as the PixiJS expansion (#502) — Pixi 8's `eventMode` and
Canvas's text-alignment values are open enumerations, and a sum-type
binding would either freeze the set or require tagged-variant codegen
that doesn't exist on the Deno-ESM backend yet (deferred to json.affine
v0.3). Consistency with existing patterns.

## Test plan

- [x] `dune build bin/main.exe` — clean (only the expected parser
warnings)
- [x] `dune runtest --force` — 356 tests pass (was 354 pre-PR; +2 from
new Canvas + Ipc modules' AOT smoke)
- [x] `tools/run_codegen_deno_tests.sh` — all 19 harnesses including the
new `canvas_smoke.harness.mjs` OK
- [ ] CI build job
- [ ] CI `tools/run_codegen_deno_tests.sh` job
- [ ] CI governance + Hypatia (6 baselines per repo CLAUDE.md may be red
— pre-existing on main, not regressions)

## Refs

- Umbrella: #446 (Tier 1 — idaptik blockers)
- Tier-1 sub-issue: #450
- Row updated: `docs/bindings-roadmap.adoc` row #8
- Pattern siblings shipped this session: #502 (PixiJS Container
expansion), #506 (Ipc.affine)
- Adjacent / future: `affinescript-dom` (gated on #255), WebGL/WebGPU
(Tier 3 axis, not surfaced as need yet)

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