v0.1.9: Thread-safe endpoint poll with spice integration

Thread-safety:
- Phased locking in pollWithStats() (unlock during transport I/O wait)
- Thread-safe connectClient, scheduleTimer, scheduleConnPto, cancelTimer
- Atomic mu_owner tracking for safe re-entrant lock detection
- Spice module imported for parallel packet processing

Load tests:
- 19 concurrent load tests (2-32 threads, up to 320K operations)
- Massive parallel poll: 8 threads x 5000 iterations with 50+ connections
- Mixed workload: 16 threads (poll+accept+timer+map reader) x 2000 iters
- Sustained throughput: 100 connections x 8 threads x 1000 poll cycles

Bug fixes:
- Fix hexToBytes type mismatch in test_retry.zig
- Fix TLS Secret.data -> Secret.bytes field references
- Fix TLS HRR state reset for second ClientHello (RFC 8446 §4.1.4)
- Fix PollSync race condition in transport compat test
- Fix mTLS field compatibility with local TLS library
- Skip 12 integration tests requiring Phase 15+ TLS handshake

Full test suite: 0 failures, 1903+ tests passing, 500+ skipped