chore: Expand test coverage for mailbox and transit

[imthaghost]

Summary

• Adds 14 new mailbox tests across protocol, state, and a full WebSocket session driving welcome → bind → allocate → claim → open → add → message routing
• Adds 4 new transit tests covering peer-wait timeout, multiple independent pairs, EOF propagation through the relay, and a 256 KiB bidirectional transfer
• No production-code behavior changes; one minor refactor in mailbox/src/server.rs split run() into bind+serve so tests can use ephemeral ports

Test plan

• cargo test -p blackhole-mailbox - 32 tests, all green
• cargo test -p blackhole-transit - 7 tests, all green
• Full workspace cargo check --workspace clean