perf(relay): index relay table for O(1) hot-path lookups

[krazyjakee]

Summary

The UDP relay hot path (UDPRelayHandler.relay) did two Array.prototype.find linear scans over the relay table on every packet — one to resolve the sender, one for the target. Cost is O(N) in active relays, so per-packet throughput collapses as the relay count grows.

This PR replaces those scans with maintained indexes, keeping the hot path O(1) regardless of table size.

Changes

• _byAddress — nested Map (address → port → entry), so the hot path looks up the sender by raw address + port with no per-packet string concatenation.
• _byPort — Map (port → entry) for the target lookup.
• Indexes are kept in sync in createRelay / freeRelay / clear (cost paid once per relay, not per packet).
• Split relay() into a thin NetAddress wrapper over a new allocation-free relayRaw(); the socket data callback now calls relayRaw directly, removing the per-packet new NetAddress. A NetAddress is still built on the (cold) drop path to preserve the drop event contract.

Benchmark

bun scripts/bench.relay.ts (added). Measures the real relay/relayRaw path (metrics + event emit intact) with a no-op socket injected, so it isolates relay-logic CPU cost rather than loopback syscalls.

Active relays	Before (linear scan)	After (indexed)	Speedup
10	1,723,328 /s	1,737,344 /s	~1x
1,000	468,080 /s	1,466,026 /s	~3x
5,000	90,678 /s	1,382,513 /s	~15x
10,000	38,710 /s	1,291,839 /s	~33x

Before: ~44x throughput collapse from 10 → 10,000 relays. After: flat ~1.3–1.7M ops/s.

Note: this microbenchmark measures relay-logic CPU cost with a no-op socket, so absolute numbers are a ceiling, not end-to-end pps (real socket.send() syscalls bound throughput lower). The point is that the bound is now independent of relay count.

Testing

• All existing spec/unit tests pass (bun test test/spec → 144 pass, 3 pre-existing skips).
• End-to-end forwarding verified through real loopback UDP sockets via the new relayRaw path.
• Added unit coverage for the new index bookkeeping in test/spec/relay/udp.relay.handler.test.ts:
  • direct relayRaw() transmit — exercises the shipped hot path itself, not just the relay(NetAddress) wrapper.
  • drop after target free — freeing a target removes it from _byPort, so a subsequent relayRaw() to that port drops (no send, drop emitted).
  • shared-address bucket integrity — two relays on one address: freeing one keeps the sibling routable; freeing the last cleans up the empty _byAddress bucket (the ports.size === 0 branch).
  • drop-event address contract — the drop event's sender is a value-equal NetAddress, which dynamic relaying keys off.