feat(compositor): scaffold macOS Metal+VideoToolbox port

Branch off the CPU-backend seam established by PR #162 and split the
compositor crate into per-platform directories. Windows behaviour is
byte-identical (81 tests pass) and the macOS stubs that follow this
commit in the stack expose the same public surface (Backend, Gpu,
Compositor, Decoder, VideoEncoder, ExportCodec, ClipSource, LiveParams,
LayerCB...) and Err from every operation.

Renames + cfg dispatch:
  - crates/compositor/src/d3d_windows.rs           (from d3d.rs)
  - crates/compositor/src/cpu_frames_windows.rs    (from cpu_frames.rs)
  - crates/compositor/src/compositor_windows.rs    (from compositor.rs)
  - crates/compositor/src/pipeline_windows.rs      (from pipeline.rs)
  - crates/compositor/src/text_windows.rs          (from text.rs)
  - crates/compositor/wrapper_windows.h            (from wrapper.h)

lib.rs cfg-dispatches 'd3d', 'cpu_frames', 'compositor', 'text' across
the per-platform modules so call-sites stay portable.

live.rs: Win32 harness (run_standalone, host_proc, wide, client_size)
moved into a #[cfg(windows)] pub mod standalone_harness with a
dev-only on Windows).

Cargo.toml: macOS deps (metal, objc, block, core-foundation) added
under [target.cfg(target_os = macos).dependencies]; windows dep
gated to cfg(windows). compositor-view-napi mirrors the gate.

crates/.cargo/config.toml: target-specific env block for macOS
(LIBCLANG_PATH not needed, MAC_FFMPEG_DIR falls back via build.rs).

build.rs: selects wrapper_windows.h vs wrapper_macos.h on
CARGO_CFG_TARGET_OS, sets bindgen --target=aarch64-apple-darwin +
-isysroot on macOS, finds MAC_FFMPEG_DIR or vendored
thirdparty/ffmpeg-n8.1.2-macos64-lgpl-shared.

Stacked on feat/d3d-warp-fallback (PR #162).

Author: openscreen-bot

.github/workflows/ci.yml

@@ -89,14 +89,60 @@ jobs:
       - uses: ./.github/actions/setup
       - run: npm run test
 
-  build:
+build:
     name: Build
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
       - uses: ./.github/actions/setup
       - run: npx vite build
 
+  # PR #189 — port macOS du compositor (Metal + VideoToolbox).
+  #
+  # Compile check on a `macos-14` runner (Apple Silicon). Verifies that the
+  # scaffold + engine-layer code added by #189 actually compiles for
+  # `aarch64-apple-darwin` — bindgen + cc on macOS produce the ffmpeg FFI
+  # bindings + the VideoToolbox context that the Rust crate statically
+  # references, and the `metal`/`objc`/`block`/`core-foundation` crates need
+  # their macOS system frameworks (Metal/CoreVideo/CoreMedia) linked.
+  #
+  # ffmpeg via Homebrew: this job installs `ffmpeg` from homebrew-core on the
+  # runner to satisfy bindgen + the cargo:rustc-link-lib lines for avformat/
+  # avcodec/avutil/swscale/swresample. NB: homebrew's `ffmpeg` is GPL-3.0 (it
+  # bundles x264/x265/svt-av1 etc.) — that is fine for a CI compile-check, but
+  # the dev/runtime story is different: `scripts/fetch-ffmpeg.mjs` documents
+  # that BtbN publishes no macOS build, so a pinned LGPL macOS dylib has to
+  # land separately before we can ship. The vendored `electron/native/bin/
+  # darwin-*/` directory is the runtime pin and is empty today.
+  #
+  # Why this job runs at all today: every commit on #189 that touches
+  # compositor_macos.rs / pipeline_macos.rs / mac_frames.rs / shaders.metal /
+  # d3d_macos.rs / text_macos.rs is unverifiable from Windows (cross-compile
+  # requires the macOS SDK + headers, which we don't have). This job is the
+  # only signal that the macOS-side Rust still type-checks.
+  rust-macos-compositor-check:
+    name: Rust check (macOS compositor)
+    runs-on: macos-14
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install Rust toolchain
+        run: |
+          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable --target aarch64-apple-darwin
+          echo "$HOME/.cargo/bin" >> "$GITHUB_PATH"
+      - name: Install ffmpeg (homebrew-core) and verify headers/libs
+        run: |
+          brew update
+          brew install ffmpeg
+          ls /opt/homebrew/opt/ffmpeg/include | head -5
+          ls /opt/homebrew/opt/ffmpeg/lib | head -5
+          /opt/homebrew/opt/ffmpeg/bin/ffmpeg -hide_banner -version | head -1
+      - name: cargo check (compositor + napi addon, aarch64-apple-darwin)
+        env:
+          MAC_FFMPEG_DIR: /opt/homebrew/opt/ffmpeg
+        run: |
+          cd crates
+          cargo check --target aarch64-apple-darwin -p openscreen-compositor -p compositor-view-napi
+
   semantic-pr:
     name: Validate PR title (semantic)
     runs-on: ubuntu-latest

crates/.cargo/config.toml

@@ -11,3 +11,10 @@
 [env]
 FFMPEG_DIR = { value = "thirdparty/ffmpeg-n8.1.2-win64-lgpl-shared", relative = true }
 LIBCLANG_PATH = "C:\\Program Files\\LLVM\\bin"
+
+# macOS : BtbN ne publie pas de build macOS (cf. scripts/fetch-ffmpeg.mjs), donc on
+# laisse `crates/compositor/build.rs` chercher MAC_FFMPEG_DIR (env var explicite posée
+# par la CI macOS ou par le dev local) ou un répertoire attendu sous `thirdparty/`.
+# LIBCLANG_PATH est inutile sur macOS (clang est dans CommandLineTools), on ne le
+# pose donc pas.
+[target.'cfg(target_os = "macos")'.env]

crates/Cargo.lock

@@ -2,7 +2,7 @@
 name = "compositor-view-napi"
 version.workspace = true
 edition.workspace = true
-description = "Addon napi-rs : expose openscreen_compositor::live::LiveView (fenêtre D3D enfant) à Electron. C'est CE crate qui produit compositor_view.node, le binaire packagé dans l'app."
+description = "Addon napi-rs : expose openscreen_compositor::live::LiveView (vue offscreen multiplateforme) à Electron. C'est CE crate qui produit compositor_view.node, le binaire packagé dans l'app."
 
 [lib]
 crate-type = ["cdylib"]
@@ -14,9 +14,13 @@ napi-derive = "2"
 openscreen-compositor.workspace = true
 anyhow.workspace = true
 
-[dependencies.windows]
+# Le crate n'utilise pas directement l'API Windows — c'est transitif via le
+# moteur de composition (qui expose la même surface publique cross-platform via
+# la ré-export cfg dans `openscreen_compositor::lib`). On garde la dépendance
+# cfg-conditionnelle pour que le crate compile sur macOS.
+[target.'cfg(windows)'.dependencies.windows]
 version = "0.58"
 features = ["Win32_Foundation"]
 
 [build-dependencies]
-napi-build = "2"
+napi-build = "2"