Compile Metal Gaussian raster from shared Slang shaders

[snkmcb]

Summary

Metal Gaussian rasterization previously compiled a separate embedded MSL implementation when creating the backend. Compile it from shared Slang position/ellipse/alpha calculations into an embedded metallib at build time, preserving the CPU-sorted reference image and color/depth/picking behavior.

• Share raster math with Vulkan and retain Metal-specific scalar stream bindings and combined color/ID outputs.
• Package MSL, reflection, artifact identities, metallib and checksum; verify generated bindings, host offsets and installed consumers.
• Provision pinned Slang in macOS CI and document Windows/Vulkan and macOS/Metal setup using the existing CMake presets.

Related issue: implements the first shared-raster step of the Metal Gaussian execution plan introduced in #108.

Impact

• Affected areas: Metal, shared Vulkan raster math, shader build/packaging, CI, tests and documentation.
• Metal source builds now require Slang 2026.8.x and Xcode's offline Metal tools. Core-only builds remain independent of both. Installed Metal consumers use the embedded library without shader search paths.
• Public APIs and execution settings are unchanged. GPU Gaussian preparation/sorting/tiling remain unsupported on Metal; preferred execution falls back and required execution rejects. The scalar stream explicitly rejects sizes beyond its 32-bit byte-address limit.
• README and the build guide separate Windows/Mac instructions, build/runtime dependencies, and Hydra/usdview host setup. No CMake preset definitions were changed.

Verification

Local runtime environment: Apple M3, macOS 15.7.9, Slang 2026.8, Xcode 16.4 Metal compiler 32023.620 with macOS SDK 15.5.

ctest --test-dir build-metal-release --output-on-failure
  23/23 passed, no skips; includes Metal image/AOV/lifetime, ABI,
  artifact identity, Vulkan Gaussian SPIR-V compilation and install consumers.

ctest --test-dir build-metal -R 'metal|install-consumer|release-metadata|gaussian-preparation' --output-on-failure
  9/9 passed, no skips.

cmake --preset core -B /tmp/merlin-core-slang-check -G Ninja -DCMAKE_BUILD_TYPE=Release
cmake --build /tmp/merlin-core-slang-check -j 6
ctest --test-dir /tmp/merlin-core-slang-check --output-on-failure
  Clean Core-only build; 17/17 passed without Slang/Metal discovery.

cmake --list-presets=all
  Documented presets exist; Markdown links/code fences and git diff --check passed.

Rebuilt hdMerlin and manually opened a local 5,834,784-particle stage in usdview with OpenUSD 26.08. Rendering completed through HgiMetal GPU-copy and the displayed image was confirmed. The private scene and local raw reports are not included in this PR.

Not run: Windows/Vulkan GPU runtime, controlled performance measurements, and a native viewport image comparison. The shared Vulkan raster entry points compile to SPIR-V locally; this does not establish Vulkan GPU runtime coverage. The complete Phase 1 measurement/host-comparison gate remains open.

Review checklist

• The change follows the contribution guidelines.
• Observable behavior has tests or a documented reason tests are not needed.
• User-facing changes are recorded under Unreleased in the changelog.
• New support claims and command examples match validated behavior, with untested platforms/gates identified above.