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/network_player_proxy.gd

@@ -1,43 +1,11 @@
 extends Node
-## Client-side proxy for the server's real PlayerNode (scripts/
-## player_spawner.gd), attached at runtime to a Node named "Player_<peer_id>"
-## under the /root/Server proxy by NetworkClient._on_connected().
+## Client-side proxy for the server's PlayerNode (scripts/player_spawner.gd),
+## attached at runtime as "Player_<own_peer_id>" under the /root/Server proxy.
 ##
-## rpc_request_input is the only one of these actually CALLED from this
-## side -- the real implementations all live on the server. But two things
-## force every @rpc method on the real PlayerNode to be mirrored here too,
-## not just the one this class uses:
-##   1. Godot's rpc_id() looks up the RPC transfer config (mode/reliability)
-##      from the LOCAL script before it will even send a call, so the method
-##      being called has to be declared locally with a matching annotation.
-##   2. Godot's multiplayer layer also verifies an "RPC node checksum" --
-##      both peers must declare the *same set* of @rpc methods at a matching
-##      path, or every RPC at that path starts failing, not just the one
-##      that's actually missing. So even rpc_request_interact/attack/
-##      set_position/set_rotation, which this class never calls, have to be
-##      declared here with matching annotations or rpc_request_input itself
-##      breaks too.
+## rpc_request_input is the only RPC the client sends. Godot resolves the RPC
+## transfer config from the LOCAL node's declaration and verifies both peers
+## declare the same @rpc set at this path, so it must match player_spawner.gd.
 
 @rpc("any_peer", "call_remote", "reliable")
-func rpc_request_input(_input_state: Dictionary, _seq: int) -> void:
-	pass  # Never invoked locally -- see the note above.
-
-
-@rpc("any_peer", "call_remote", "reliable")
-func rpc_request_interact(_target_id: int) -> void:
-	pass
-
-
-@rpc("any_peer", "call_remote", "reliable")
-func rpc_request_attack(_target_id: int, _aim_dir: Vector3) -> void:
-	pass
-
-
-@rpc("authority", "call_remote", "reliable")
-func rpc_set_position(_pos: Vector3) -> void:
-	pass
-
-
-@rpc("authority", "call_remote", "reliable")
-func rpc_set_rotation(_rot: Vector3) -> void:
-	pass
+func rpc_request_input(_input: Dictionary, _seq: int) -> void:
+	pass  # Real implementation is on the server.