macOS native parity + GPU tone curve shader fix

[myyc]

macOS parity

Convert the macOS raw processor to the wrapper pattern (same as Linux), which was the last standalone drift point after the Linux cleanup branch. Before: macos/raw_processor/raw_processor.c was a 250-line standalone copy ~60 lines out of sync with canonical. Most importantly it lacked raw_processor_set_highlight_mode, so any macOS build after the Linux refactor would crash at image load when the Dart FFI looked up the missing symbol.

• Add macos/raw_processor/raw_processor_wrapper.c (one-liner that #includes ../../lib/ffi/raw/raw_processor_common.c).
• Delete macos/raw_processor/raw_processor.{c,h}.
• Update macos/build.sh and macos/Makefile to compile the wrapper and pass -I../lib/ffi/raw for the canonical header.
• Update Xcode project input-paths so the "Build Native Libraries" phase re-runs on wrapper/canonical changes.
• Update CLAUDE.md — macOS no longer needs special mirroring.

GPU tone curve fixes

Audit of ROADMAP's "GPU tone curve pending" claim: the shader code is all there, but there were two real bugs that a CPU↔GPU comparison test would have caught:

1. Operation order was reversed from CPU. The shader applied master RGB curve first then per-channel curves; CPU does per-channel first. Non-commutative for non-identity curves — outputs differed by several LSBs.
2. Float→int truncation when indexing the byte LUT: sRGB 0.50196 (byte 128) got truncated to index 127 instead of rounded to 128. Off by one at every quantisation boundary.

Both fixed in linux/vulkan_processor/shaders/image_process.comp. Added a CPU↔GPU comparison test with an S-curve + distinct per-channel curves; CPU and GPU now agree to within 1 LSB (measured: maxDiff 0, avgDiff 0.00 on the full test fixture).

Tests

31 native tests pass (was 30), 66 unit tests pass, dart analyze clean.