MSL: function-scope workgroup variables (PR #77, @georgebuilds)

Fixed

• MSL workgroup vars at function-body scope — workgroup (threadgroup) variables
  are now declared inside the kernel function body instead of as entry-point parameters.
  The parameter form requires the host to call setThreadgroupMemoryLength:atIndex: before
  dispatch; our Pure Go Metal HAL does not, causing workgroup memory to silently no-op.
  Function-scope declarations are legal MSL, statically sized by the compiler, and need
  no host-side setup. Verified on Apple M3 Max with Metal 3.1 runtime (@lkmavi).

• MSL per-entry-point workgroup zero-init filtering — writeWorkgroupZeroInit now
  filters by entry-point usage (matching Rust naga), so only workgroup vars actually used
  by the entry point are declared and zeroed.

Changed

• CONTRIBUTING.md — snapshot test infrastructure docs: golden file naming convention
  (1:1 with Rust naga test suite), reference allow-list, adding new test shaders.
  Updated project structure and testing commands.

Quality gates

• TestRustReference: ALL PASS (100%)
• SPIR-V validation: 172/172 (100%)
• golangci-lint: 0 issues
• CI: 11/11 checks pass (Linux, macOS, Windows)
• Hardware verified: Apple M3 Max, Metal 3.1 (@lkmavi)