test: add transport layer tests (Phase 2) — node/ coverage 42% to 63.5%
13 tests covering all Phase 2 TODO items: reusable test pair helper,
full handshake with challenge-response verification, protocol version
rejection, allowlist rejection, SMSG encrypted message round-trip,
message deduplication, rate limiting (100 burst verified), MaxConns
enforcement, keepalive timeout cleanup, graceful disconnect, and
concurrent sends with race detection clean.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>