Add native macOS Metal monitoring support

[CodingInAVan]

Summary

• Add an Apple Metal backend for native macOS GPU monitoring.
• Add macOS build and monitor helper scripts with Homebrew OpenSSL detection.
• Integrate Metal with automatic and explicit backend selection.
• Emit telemetry_backend: metal and profiling_engine: metal.none.
• Report public Metal device metadata, including architecture, registry ID,
  unified-memory status, working-set limits, GPU families, and counter sets.
• Distinguish current-process Metal allocations from unavailable global VRAM
  and NVML-style metrics.
• Preserve existing device batch columns for wire compatibility.
• Add Metal collector and wire-contract tests.
• Document native macOS monitor setup and telemetry semantics.

Validation

• ./build-macos.sh --monitor
• 37/37 WireContract.* tests passed.
• Native MetalCollector.* hardware test passed on Apple M2.
• Native monitor smoke test emitted the expected Metal metadata and shut down
  cleanly.
• Full non-upload suite: 565 passed, 1 skipped, 1 unrelated failure.

Known Test Issue

SourceCapturePolicyTest.RejectsSymlinkThatEscapesTheApprovedRoot fails on
macOS because /var canonicalizes to /private/var, resulting in
OutsideApprovedRoots instead of SymlinkEscape. This branch does not modify
the source-capture implementation or test.