P13-T3: Implement multi-client transport and JSON-RPC multiplexing

[SoundBlaster]

Description

Implements P13-T3: full UnixSocketServer in src/mcpbridge_wrapper/broker/transport.py, replacing the P13-T1 stub. The server accepts concurrent Unix domain socket connections from MCP clients and multiplexes their JSON-RPC traffic through the single upstream xcrun mcpbridge subprocess owned by BrokerDaemon.

Key changes:

• transport.py — full UnixSocketServer implementation: client accept loop, per-session handlers, request ID remapping (session_id << 20 | original_id), upstream write, response routing, notification broadcast, graceful shutdown with -32001 drain, and queue TTL enforcement during reconnection.
• daemon.py — integrates UnixSocketServer as an optional transport parameter; starts/stops transport in start()/stop(); routes upstream stdout lines through transport.route_upstream_response().
• test_broker_transport.py — 32 new tests covering concurrent clients, ID remapping, error isolation, reconnect TTL, graceful stop.
• test_broker_stubs.py — replaced NotImplementedError assertions with instantiation tests (stubs now live).

Validation report: SPECS/ARCHIVE/P13-T3_Implement_multi-client_transport_and_JSON-RPC_multiplexing/P13-T3_Validation_Report.md

Type of Change

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

Quality Gates

Before submitting, ensure all quality gates pass:

make check

Or run individually:

• make test - All tests pass with ≥90% coverage (550 passed, 5 skipped; 93.6% coverage)
• make lint - No linting errors (ruff check src/ — all checks passed)
• make format - Code is properly formatted
• make typecheck - Type checking passes (mypy src/ — no issues in 18 files)
• make doccheck - Documentation is synced with DocC (if docs changed)

Documentation Sync

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

Testing

• Added/updated tests for new functionality (32 new tests in test_broker_transport.py)
• All tests pass locally (550 passed, 5 skipped)
• Manually tested the changes (integration with live xcrun mcpbridge requires Xcode 26.3+)

Checklist

• Code follows the project's style guidelines
• Self-review completed
• Comments added for complex code (ID remapping logic documented in module docstring and inline)
• Documentation updated (if needed) — N/A
• No new warnings generated
• PR title is descriptive