Metal device, command queue, and CAMetalLayer setup #289

Implements issue 3mi434slnsv2g

Changes #

• metal module in the platform crate with Metal framework FFI bindings:
  • MTLCreateSystemDefaultDevice() for obtaining the default GPU
  • Device, CommandQueue, CommandBuffer, RenderCommandEncoder wrapper types
  • MetalLayer (CAMetalLayer) with drawable size, pixel format, and device configuration
  • make_clear_pass_descriptor() helper for setting up render passes
  • drawable_texture() to extract the texture from a CAMetalDrawable
• MetalView in appkit.rs: custom WeMetalView NSView subclass using CAMetalLayer
  • Overrides makeBackingLayer to provide CAMetalLayer
  • Overrides updateLayer to render a clear-to-color pass via Metal
  • Full event handler support (key, mouse, scroll)
• Browser main.rs updated to try MetalView first, fall back to BitmapView
• Double-buffered presentation via CAMetalLayer.nextDrawable
• Drawable size updates on window resize

Tests #

6 new Metal tests covering:

• ClearColor and MetalSize struct construction
• Pixel format constants
• System default device creation
• Command queue creation
• CAMetalLayer creation and configuration

Dependencies #

Links Metal.framework and QuartzCore.framework. No external crate dependencies. All new code is in the platform crate where unsafe is permitted.

Phase 12 — Metal GPU Compositor (1/7)