fix(q): harden codec and connection handling

[belowzeroff]

Summary

• Reject unsupported nested values before serialization, check encoded sizes, and bound recursive decoding depth.
• Preserve keyed-table dictionaries when encoding and decoding, and validate Q handles before closing or narrowing them to socket descriptors.
• Keep poll writes bounded, queue server responses through writable events, return a complete response before processing a following FIN, and shut down blocking streams after transport or framing failures.
• Close Q listeners when their poll is destroyed and add regression coverage for the reported failures.

User-facing behavior

Malformed or excessively nested Q messages now return an error instead of corrupting memory or exhausting the process stack. Poll-attached sends honor the configured timeout, slow readers no longer block the shared event loop, and timed-out blocking connections cannot deliver a stale response to the next request. Keyed tables retain their Q dictionary wire type, and closing an invalid or oversized handle cannot close an unrelated descriptor.

Tests

• Full test/run.sh integration suite, including real-q interoperability and poll push tests.
• Full integration suite built with AddressSanitizer and UndefinedBehaviorSanitizer (ASAN_OPTIONS=detect_leaks=0).