[codex] Compile parametric brush shaders into metallib

[muukii]

Summary

• Move parametric brush Core Image kernels from copied .metal.txt source to build-compiled .metal sources.
• Move the live brush-mask render shader into BrightroomParametric so it is compiled into the same default.metallib.
• Share brush falloff through BrushStampFalloff.metalh, loaded by both the Core Image kernel and live Metal render shader.
• Load kernels via CIColorKernel(functionName:fromMetalLibraryData:) and live render functions via MTLDevice.makeLibrary(URL:).

Why

The previous resource-copy/runtime-compile setup existed to avoid missing brushStampAlpha during Metal compilation. Now the shared falloff is a real Metal header included by both compiled sources, so the package can ship compiled Metal instead of text resources.

Validation

• BrightroomUI simulator build via XcodeBuildMCP: succeeded.
• BrightroomEngineTests targeted tests via XcodeBuildMCP: 4 passed.
  • BrushStampFalloffTests
  • BrushMaskRasterizerParityTests
• Confirmed build output contains Brightroom_BrightroomParametric.bundle/default.metallib and build log runs CompileMetalFile + MetalLink for the new .metal sources.