Document Metal Gaussian GPU execution strategy

[snkmcb]

Summary

Record the proposed Metal Gaussian GPU execution strategy in a standalone design document, with links from the shared Gaussian design and backlog. The plan sequences measurement and shared Slang shaders, persistent attributes and GPU execution through raster, then tile rasterization and Apple GPU tuning.

It defines acceptance gates, CPU reference/fallback behavior, image and performance evidence, and deferred optimizations. Implementation support remains documented separately from proposed work.

Related PR: #107.

Impact

• Documentation only: three Markdown files.
• No code, build, package, API or runtime changes.
• Preserves the existing shared design as the pipeline contract and the roadmap as the unfinished-work index.

Verification

• Local relative Markdown links resolve.
• git diff --check passes.
• Rendering tests are not rerun for documentation-only changes. The implementation in Add Metal Gaussian rendering for native viewport and usdview #107 passed hosted CI and self-hosted Vulkan/Hydra GPU CI.

Review checklist

• The change follows the contribution guidelines.
• Documentation only; no new runtime behavior requires tests.
• Proposed scope is distinguished from shipped support.
• Existing design/backlog navigation links to the standalone plan.