Add Metal Gaussian rendering for native viewport and usdview

[snkmcb]

Summary

Gaussian-only USD stages opened as an empty view with the Metal backend. Add CPU-sorted Gaussian rendering on Metal so these stages render in the native viewport and usdview. The shared projection, covariance, SH evaluation and sorting implementation now lives in RenderExtraction; Vulkan retains its existing raster path.

Metal draws alpha-blended ellipses against opaque mesh depth and writes resource/particle picking IDs. Immutable prepared buffers are reused across unchanged frames and retained across in-flight edits. Also fix native Hydra regression logging reading an unrequested depth buffer, and enable Objective-C at the top-level macOS configure scope.

Related issue: none.

Impact

• Areas: Core extraction, Metal, Vulkan build wiring, Hydra, viewport, tests, documentation and GPU capability CI.
• Adds a backend-neutral Gaussian preparation header to the installed extraction package. No renderer settings or GPU shader ABI changes.
• Metal preparation/sorting remains CPU-based: prefer falls back, require rejects. Camera/scene changes prepare a new stream; GPU preparation and tile rasterization remain Vulkan-only.
• Documents the new Metal support and adds Gaussian resource/preparation/raster tests to the Windows self-hosted GPU CI selection.

Verification

Local Apple GPU / Release / OpenUSD 26.08:

• Eight source/runtime tests passed: Metal Gaussian image/AOV/composition/cache/in-flight tests, existing Metal backend, native USD Gaussian viewport with screenshot/reference/resize/logging, Gaussian preparation/resource, extraction/world, and backend boundaries.
• Install-tree consumer passed.
• Hydra discovery, render-buffer and HgiMetal bridge tests passed.
• A local 5,834,784-particle USD stage rendered successfully in the native Metal viewport and usdview; usdview reported hgi_metal_transfer_mode=gpu-copy and zero validation messages. The user confirmed both displays. Native capture completed three frames successfully. The private scene/image is not included in this PR.
• git diff --check passed.

Commands used:

cmake --build build/cy2026-macos-arm64-py313-usd--viewport-usd --target merlin-viewport hdMerlin merlin-metal-gaussian-tests merlin-metal-backend-tests merlin-gaussian-preparation-tests -j 6
ctest --test-dir build/cy2026-macos-arm64-py313-usd--viewport-usd -R '^merlin-(metal-(gaussian|backend)|viewport-metal-gaussian|gaussian-(resource|preparation)|render-(world|extraction)|backend-boundaries)$' --output-on-failure
ctest --test-dir build/cy2026-macos-arm64-py313-usd--viewport-usd -R '^merlin-install-consumer$' --output-on-failure
ctest --test-dir build/cy2026-macos-arm64-py313-usd--viewport-usd -R '^merlin-hydra2-(discovery|render-buffer|hgi-metal-bridge)$' --output-on-failure

Windows Vulkan/HgiVulkan cannot be exercised on this Mac. Self-hosted GPU CI passed for commit 7305ec8 in Debug/Release and OpenUSD 26.05/26.08. Hosted Core/Metal/MaterialX CI passed all 12 jobs for the same commit.

Review checklist

• The change follows the contribution guidelines.
• Observable behavior has tests.
• User-facing changes are recorded under Unreleased.
• Support claims and examples match validated behavior; GPU-driven Metal support is not claimed.