feat(compositor): compositor_macos engine — render targets, MSL pipeline, first-pass compose_frame

Wires up the Metal compositor engine. new_sized now allocates render
targets and compiles the MSL shader library at compositor construction:

Render targets (MTLStorageMode::Private for GPU-only, Shared for readback):
  - rt (RGBA8)         — main composition target
  - rt_read (RGBA8)     — CPU-readable mirror for preview live
  - nv12_y (R8)        — Y plane of the NV12 output
  - nv12_uv (RG8)       — UV plane of the NV12 output
  - nv12_read_y/_uv     — CPU-readable mirrors for encode

MSL library: include_str! of shaders.metal, compiled at new_sized via
MTLDevice.new_library_with_source. Function lookups for vs_main, vs_fs,
ps_main, ps_y, ps_uv, ps_tex.

Pipeline states:
  - pipeline_main       — vs_main + ps_main, RGBA8 color attachment,
                          premultiplied alpha blend (SrcOne / DstOneMinusSrcAlpha)
  - pipeline_fs_y       — vs_fs + ps_y, R8 color attachment
  - pipeline_fs_uv      — vs_fs + ps_uv, RG8 color attachment
  - pipeline_fs_tex     — vs_fs + ps_tex, RGBA8 color attachment

Engine methods (first-pass implementation):
  compose_frame(screen, _webcam, _frame, _cfg)
    — first-pass blit: full-canvas (LayerCB mode=0) of the screen frame's
      Y+UV textures (via nv12_srvs) into the RGBA8 RT. The webcam is
      ignored, the layered composition (rounded corners, shadows, blur
      pyramid, motion blur, timeline interpolation) is left to follow-up
      commits that wire each LayerCB quad individually. This passes the
      encode / preview-loop test path: a full-canvas video frame lands
      on the RT.
    — Empty screen → clear_rt (RT to black).

  render_nv12()
    — Two-pass fullscreen conversion: pass 1 (vs_fs + ps_y) writes Y to
      nv12_y; pass 2 (vs_fs + ps_uv) writes UV to nv12_uv. Same BT.709
      limited RGB→Y'CbCr pipeline as compositor_windows::render_nv12.

  rgb_to_nv12(out_tex, slice) / rgb_to_nv12_scaled(...)
    — First-pass: route to render_nv12 (writes the internal nv12_y/_uv).
      The cross-engine zero-copy path to an external encoder-owned
      CVPixelBuffer lands with the encodeur VT commit.

  readback_direct()
    — Synchronous get_bytes on rt_read → Vec<u8> RGBA8.

  readback_resized(target_w, target_h)
    — First-pass: routes to readback_direct; the resize is left to a
      follow-up commit.

  read_nv12_scaled(target_w, target_h, dst_y, pitch_y, dst_uv, pitch_uv)
    — get_bytes from nv12_read_y/_uv → caller-provided AVFrame planes.

compose_frame_mb, dump_nv12, dump_raw, blit_to remain Err/no-op — they
exercise engine layers that are not yet wired (the layer-by-layer
composition that compose_frame_mb needs).

Helper: Layer (repr(C, align(16))) matches the HLSL/MSL cbuffer layout
field-for-field (each float4 naturally 16-byte aligned, float2 inside
float4 padded the same way on both sides). Uploaded via
set_fragment_bytes (the VS doesn't read it in full-canvas mode).

Author: openscreen-bot