feat: wasmbox client backend (js/wasm) — one app runs native AND in wasmdesk

[tannevaled]

Summary

Adds a third, additive backend behind the existing Open/Run/Backend
API: a wasmbox external-client backend (GOOS=js/wasm) that lets a
go-widgets app run as a client of the wasmdesk/wasmbox
browser compositor, speaking wasmbox's existing client wire protocol
(docs/protocol.md). One go-widgets application now runs unchanged natively
(X11/Wayland) AND inside wasmdesk.

The wasmdesk/wasmbox repository is not modified — this is purely a
client-side backend plus a worker shim; the compositor hosts it unchanged.

What's here

• internal/wasmbox/protocol.go — sovereign, transport-agnostic codec:
  hello/welcome/commit/input/set_title/request_close/closed
  encode+decode, SAB damage-rectangle maths, and input → toolkit.Event
  mapping (mirroring the X11/Wayland backends: mousedown→Click, drag-vs-move,
  wheel→Scroll±1, printable key→KeyDown+Char, named key→KeyDown, modifiers
  dropped). No syscall/js dependency — builds on every GOOS, 100%-covered.
• internal/wasmbox/client_js.go (//go:build js && wasm) — the
  syscall/js transport implementing Backend: allocates the w×h×4
  SharedArrayBuffer, posts hello (SAB handle + stride) over the per-client
  MessagePort, awaits welcome, paints the root into the SAB and posts
  commit — whole-surface, or (a DamageRenderer root, e.g.
  toolkit/scene.HostRoot) only the damaged rectangles. Drains input,
  handles set_title/request_close/closed.
• open_js.go (//go:build js && wasm) — Open returns the wasmbox
  backend; the non-linux stub is re-gated !linux && !(js && wasm) so the js
  path is real (no longer ErrUnsupported).
• clients/gowidgets/{worker.js,build.sh} + cmd/gowidgetsclient — a
  minimal worker shim and a tiny go-widgets app (VBox + Label + Button) built to
  wasm that a wasmbox compositor spawns via wasmboxSpawnExternal.

Tests + verification

• Protocol codec: 100% statement coverage (internal/wasmbox, verified with
  -race, CGO=1) — factored so it is pure and testable on any GOOS.

• CI: added GOOS=js GOARCH=wasm to the cross-build (build + vet) matrix and
  extended the 100% coverage gate to internal/wasmbox. Existing X11 / Wayland /
  codec coverage + the full cross-build matrix stay green.

• Live browser proof (headless Chromium via Playwright): test/probe-wasmbox.mjs
  loads test/harness.html — a protocol-faithful wasmbox compositor stand-in —
  served by wasmbox's unmodified cmd/serve (COOP/COEP for
  SharedArrayBuffer). It asserts the go-widgets tree rendered (41 600
  non-background canvas px) and that an injected click round-tripped to a
  toolkit.Event (a new commit; counter 0→1; label pixels changed).
  Screenshot committed: test/wasmbox-live-proof-2026-08-09.png.

  Note: the proof runs against a protocol-faithful harness compositor (the
  exact client-facing wire), not the full Ruby wasmbox desktop — chosen for a
  reliable, reviewable capture. The transport it exercises is the real one.

Horodate: 2026-08-09 21:12 CEST

🤖 Generated with Claude Code