PC3 Phase 2.1: server-authoritative sim + prediction/reconciliation with replay

Made the multiplayer core actually work (it had never run a real sim):

- project.godot: physics pinned to 30 Hz -- the shared sim/snapshot/prediction rate.
- server.gd: loads the world's level (real ground + collision), spawns a
  server Player per peer at the level's PlayerSpawn, broadcasts a full
  snapshot every physics frame with each peer's state AND last_seq (the last
  input seq applied for that peer). Accepts `--port/--world/--status-file`
  args (status file = player count for a future idle reaper); defaults keep
  `godot res://scenes/server.tscn` working standalone.
- player_spawner.gd (server Player): rewrote -- drains its whole input queue
  each physics frame in seq order, integrates gravity/walk/jump with the
  shared constants, move_and_slide(), tracks last_applied_seq. No more no-op
  process_server_input().
- network_client.gd: rewrote reconciliation -- drops inputs the server has
  acked (last_seq), re-simulates the still-in-flight ones from the
  authoritative state, snaps only on real drift. Exposes get_remote_states()
  (peer_id -> latest snapshot state) for the game to render other players.
  predict_and_send() now also sends body yaw.
- proxies trimmed to the RPC set the rewrite actually uses.
- scripts/mp_test_client.gd + run_multiplayer_2p_test.sh: two real client
  processes vs one real server -- client A sees client B and its own
  movement round-trips. PASS.
- run_checkpoint_tests.py: also run the 2p shell test.

smoke_test_network_client 4/4, player 15/15, avatar_in_world 10/10, all green.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015jtxAiu5dAW4WxCNdGkrkJ

scripts/mp_test_client.gd

@@ -0,0 +1,58 @@
+extends SceneTree
+## One client process for the 2-player multiplayer test
+## (scripts/run_multiplayer_2p_test.sh). Two real OS processes = real network.
+##
+##   godot --headless --path . --script res://scripts/mp_test_client.gd -- --role a --port 7777
+##
+## role "a": moves forward for ~2 s, then prints
+##           RESULT a remotes=<n> moved=<bool>  and exits 0 iff it both moved
+##           and saw >=1 remote player.
+## role "b": connects, holds still for ~4 s, prints  RESULT b connected  exits 0.
+
+var role := "b"
+var port := 7777
+
+
+func _initialize() -> void:
+	var args := OS.get_cmdline_user_args()
+	for i in args.size():
+		if args[i] == "--role" and i + 1 < args.size():
+			role = args[i + 1]
+		elif args[i] == "--port" and i + 1 < args.size():
+			port = int(args[i + 1])
+	_run()
+
+
+func _run() -> void:
+	await process_frame
+	var net := NetworkClient.new()
+	root.add_child(net)
+	var ok: bool = await net.connect_to_server("127.0.0.1", port)
+	if not ok:
+		print("RESULT %s connect_failed" % role)
+		quit(1)
+		return
+	for i in 15:
+		await process_frame
+
+	if role == "b":
+		# Hold still and stay connected long enough for A to boot, connect,
+		# and observe us (A's Godot startup alone is a couple of seconds).
+		print("RESULT b connected")
+		for i in 1200:
+			await process_frame
+		net.disconnect_from_server()
+		quit(0)
+		return
+
+	# role a: move forward, watch for a remote and our own displacement.
+	var start: Vector3 = net.position
+	var max_remotes := 0
+	for i in 90:
+		net.predict_and_send(1.0 / 30.0, 0.0, -1.0, false, 0.0)
+		await process_frame
+		max_remotes = maxi(max_remotes, net.get_remote_states().size())
+	var moved: bool = net.position.distance_to(start) > 1.0
+	print("RESULT a remotes=%d moved=%s" % [max_remotes, str(moved)])
+	net.disconnect_from_server()
+	quit(0 if (moved and max_remotes >= 1) else 1)