Fixed

• Browser/Rust Surface missing methods — PresentPixels, WritePixels,
  SetPrepareFrame, and SetPresentsWithTransaction now exist on all backends.
  Missing methods return clear errors or no-op, matching ADR-047 (unified public
  API). Fixes WASM build failure introduced by gogpu PR #370. (#281)

• Metal checkptr abort under -race — ObjC block callback trampolines used
  unsafe.Pointer(blockPtr + 32) which violates Go's pointer provenance rules.
  Replaced with unsafe.Add(unsafe.Pointer(blockPtr), 32) (Go 1.17+) at all 4
  block callback locations. go test -race now works on Metal compute. (#280)

Added

• Compile-time Surface API contract — anonymous interface assertion
  (var _ interface{...} = (*Surface)(nil)) in each backend file enforces that
  all public methods exist at compile time (ADR-047 enforcement).

• CI cross-compile job — GOOS=js GOARCH=wasm (browser) and -tags=rust
  (Rust FFI) builds added to CI pipeline. Missing Surface methods now fail the
  PR, not the downstream consumer.

CI

• Android SDK setup: use pre-installed SDK instead of third-party action (v3/v4 both crash on Preview license)
• goffi version check updated v0.6.1 → v0.6.2 in Android preview script
• DX12 command.go formatting fixed