feat(platform): cross-platform desktop adapters for macOS Metal and Linux (M12)

[BerryUIKI]

Summary

This PR implements Milestone M12: Cross-Platform Desktop Adapters (macOS Metal MPS & Linux).

Key Capabilities

• Cross-Platform Single-Instance Mutex:
  • Implements reliable, zero-dependency Unix domain socket single-instance locking in \launcher/src/single_instance.rs\ on non-Windows targets (macOS and Linux).
  • Handles stale socket detection, automatic unlinking on drop, and cross-platform unit tests.
• Cross-Platform Browser Launcher:
  • Abstracts workspace opening across Windows (\cmd /c start), macOS (\open), and Linux (\xdg-open).
• Hermetic Runtime Path Resolution:
  • Adapts directory structures, virtual environments, and python binary resolution for Unix (
    untime/python/bin/python, \�ackend/.venv/bin/python) and Windows.
• Platform Portability Architecture Guide:
  • Detailed reference documentation in \docs/PLATFORM_PORTABILITY.md.

Automated Verification

• Backend pytest: 103 passed (including 4 new tests in \�ackend/tests/test_m12_cross_platform.py)
• Frontend: TypeScript typecheck and Vite build passed cleanly
• Launcher: \cargo test\ (3 passed, including cross-platform single instance test)