feat(multiplayer): task 2.8 net_sim.gd, close out Phase 2

New NetSim autoload: seeded, CLI-driven (--net-sim-latency/-jitter/-loss/-dup)
latency/jitter/loss/duplicate decorator, a true no-op passthrough unless a
flag is set. Wraps MatchSim.send_input/send_snapshot per the design doc's
scope, plus NetworkManager's ping/pong so the already-tested RTT/clock
measurement becomes the acceptance signal for "raises observed RTT" without
waiting on Phase 3's per-peer snapshot echo.

Two real bugs found while building and verifying this against Phase 2's own
milestone gate (a real match under --net-sim-latency 80 --net-sim-jitter
20, not just LAN): a timestamp captured inside a delayed RPC closure
silently ate that side's own added delay out of the round-trip
measurement instead of adding to it; and a delayed send whose target
disconnected (or whose own process had already shut down) during the hold
threw RPC errors, since the existing get_peers() filtering only checked
validity at schedule time. Fixed by capturing timestamps before handing
off to NetSim, and by having NetSim re-validate the target at fire time.

Phase 2's milestone gate now passes for real: a full 1v1 under simulated
80ms latency / 20ms jitter still shows clean server-authoritative
movement and zero RPC errors. Full Phase 1 + Phase 2 regression suite
re-verified clean with NetSim present but inactive.

Game/scripts/match_sim.gd

@@ -48,11 +48,13 @@ func request_match_config() -> void:
 
 
 func send_input(bytes: PackedByteArray) -> void:
-	_recv_input.rpc_id(1, bytes)
+	# bytes is already fully packed (any timestamps it carries are already
+	# fixed), so wrapping the dispatch itself is enough — task 2.8.
+	NetSim.send(func() -> void: _recv_input.rpc_id(1, bytes), 1)
 
 
 func send_snapshot(peer_id: int, bytes: PackedByteArray) -> void:
-	_snapshot.rpc_id(peer_id, bytes)
+	NetSim.send(func() -> void: _snapshot.rpc_id(peer_id, bytes), peer_id)
 
 
 func send_score_update(score: Dictionary) -> void: