qkc/slave 07: add SlaveServer runtime and Python interop validation

[iteyelmp]

Final PR in the Slave migration series.

Adds the SlaveServer runtime that integrates MasterConn, PeerConn,
XshardConn, and XshardPool into a single lifecycle-managed process.

Also adds end-to-end Python interoperability validation using a real
pyquarkchain process over TCP, replacing fixture-only compatibility
tests.

Changes

SlaveServer Runtime

• Add SlaveServer lifecycle management (Start/Stop)
• Add accept loop and graceful shutdown handling
• Wire MasterConn, PeerConn, XshardConn, and XshardPool
• Add master-driven peer/xshard connection lifecycle

Python Interop Validation

• Launch real pyquarkchain helper process
• Exchange protocol messages over TCP
• Validate:
  • Master → Slave ping/pong
  • Peer lifecycle
  • Xshard routing

Requires PYQUARKCHAIN pointing to a valid pyquarkchain checkout.

Cleanup

• Remove obsolete fixture-only protocol tests
• Remove unused pyproto helpers

Additional Fixes

• Improve connection lifecycle cleanup
• Replace timing-based tests with condition-based synchronization
• Tighten interop environment validation