Add an end-to-end TCP acceptance harness (issue #15) (#20)

Stand up the real AuthoritativeServer behind the real TCP transport on a
loopback port and drive real SyntheticProtocolClient sockets through it, so the
matrix is exercised over an actual connection rather than in-memory fakes.

Covers the baseline-protocol, TCP-compatibility, and interest-management
sections: welcome->hello->sessionReady handshake, unique/non-spoofable
server-owned ids, malformed and oversized rejection before mutation, rate-limit
tripping, transform/playerState/worldState relay, action events not
replay-cached to late joiners, disconnect/reconnect leaving no stale session,
same-cell relay vs distant interest filtering, multi-client no cross-cell spam,
and handshake-timeout reaping.

The harness runs for real wherever loopback TCP can bind (dev machines, the
self-hosted runner) and skips cleanly otherwise. Wired into a self-hosted CI
job gated by the runtime-policy guard. 60s idle-timeout, 32/64-client load /
packet-loss / reconnect-churn, and GNS sections remain follow-ups.

.gitignore

@@ -7,6 +7,8 @@ server/obj/
 server/publish/
 server/tests/bin/
 server/tests/obj/
+server/acceptance/bin/
+server/acceptance/obj/
 cmake-build-*/
 *.exe
 *.dll