P13-T2: Implement persistent broker daemon with single upstream Xcode bridge

[SoundBlaster]

Description

Implements BrokerDaemon in src/mcpbridge_wrapper/broker/daemon.py, replacing the P13-T1 stub with a fully working persistent broker daemon that owns a single xcrun mcpbridge upstream subprocess.

Key capabilities:

• PID-file locking prevents duplicate instances; stale-lock recovery handles crashed processes
• Exponential-backoff reconnection (0s, 1s, 2s, … min(2^n, cap)s) when upstream crashes
• Graceful shutdown: drains read task, closes upstream stdin, waits with timeout, kills if needed, removes PID/socket files
• status() method returns {state, pid, upstream_pid} for health monitoring
• run_forever() registers SIGTERM/SIGINT handlers and blocks until stopped

Also updates test_broker_stubs.py to remove now-invalid NotImplementedError assertions from P13-T1.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactoring
• CI/CD improvement

Quality Gates

• make test - All tests pass with ≥90% coverage
  • 485 unit tests pass; broker module coverage 93.2%
  • 26 new tests in tests/unit/test_broker_daemon.py
• make lint - No linting errors (ruff check src/ clean)
• make format - N/A (ruff auto-fix applied during development)
• make typecheck - mypy not configured as hard gate
• make doccheck - No docs/ changes

Documentation Sync

• Documentation changes are synced with DocC catalog (or N/A) — no docs/ changes

Testing

• Added/updated tests for new functionality (tests/unit/test_broker_daemon.py, 26 tests)
• All tests pass locally (pytest tests/unit/ — 485 passed)
• Manually tested the changes — integration with live xcrun mcpbridge deferred to P13-T5

Checklist

• Code follows the project's style guidelines
• Self-review completed (see SPECS/ARCHIVE/_Historical/REVIEW_P13-T2_broker_daemon.md)
• Comments added for complex code (lifecycle state transitions, ID remapping hook)
• Documentation updated (if needed) — user-facing docs deferred to P13-T6
• No new warnings generated
• PR title is descriptive

Validation Report

SPECS/ARCHIVE/P13-T2_Implement_persistent_broker_daemon/P13-T2_Validation_Report.md — verdict: PASS

Follow-up Tasks Added

• FU-P13-T2-1 (P3): Replace run_forever() polling loop with asyncio.Event-based wait
• FU-P13-T2-2 (P3): Move PID file write to after successful upstream launch