feat(cg): use IOSurface with single-buffering

Back the CoreGraphics buffer with an IOSurface instead of CGDataProvider.
IOSurface is shared zero-copy with the compositor, avoiding the internal
copy QuartzCore did on present.

- keep single-buffer model; block on `is_in_use` in next_buffer before
  writing, since the compositor reads the surface directly (no internal
  copy to rely on)
- stride uses surface.bytes_per_row (may be padded for alignment)
- present sets CALayer.contents to the IOSurface, disabling actions to
  avoid the contents fade
- scale content to fill via kCAGravityResize + nearest-neighbour filters
- IOSurface supports Opaque|Premultiplied only; update supports_alpha_mode
  and AlphaMode platform docs accordingly
- swap deps: drop CGDataProvider/CGImage, add objc2-io-surface + CF
  Number/String/Dictionary features

Replaces the branch's earlier triple/unbounded-buffer commits; upstream
moved to single-buffering (rust-windowing#343) while rust-windowing#329 kept multi-buffering.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: flukejones

Cargo.toml

@@ -96,14 +96,14 @@ libc = "0.2.180"
 objc2-core-graphics = { version = "0.3.2", default-features = false, features = [
     "std",
     "objc2",
-    "libc",
     "CGColorSpace",
-    "CGDataProvider",
-    "CGImage",
 ] }
 objc2-core-foundation = { version = "0.3.2", default-features = false, features = [
     "std",
     "CFCGTypes",
+    "CFNumber",
+    "CFString",
+    "CFDictionary",
 ] }
 objc2-foundation = { version = "0.3.2", default-features = false, features = [
     "std",
@@ -114,14 +114,21 @@ objc2-foundation = { version = "0.3.2", default-features = false, features = [
     "NSString",
     "NSThread",
     "NSValue",
-    "NSNull",
 ] }
 objc2-quartz-core = { version = "0.3.2", default-features = false, features = [
     "std",
     "objc2-core-foundation",
     "CALayer",
     "CATransaction",
 ] }
+objc2-io-surface = { version = "0.3.2", default-features = false, features = [
+    "std",
+    "libc",
+    "objc2",
+    "objc2-core-foundation",
+    "IOSurfaceRef",
+    "IOSurfaceTypes",
+] }
 
 # Web dependencies.
 [target.'cfg(target_family = "wasm")'.dependencies]