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_receiver_proxy.gd

@@ -1,38 +1,15 @@
 extends Node
-## Client-side proxy node, attached at runtime to a Node named "Server" at
-## the SceneTree root by NetworkClient.connect_to_server(). Exists purely so
-## Godot's RPC path-matching finds a receiver for _receive_snapshot -- see
-## network_client.gd's class docstring for the full explanation of why this
-## has to exist at this exact path. Not a real server in any sense.
-##
-## Godot's multiplayer layer 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 (not just the mismatched one) starts failing with
-## "rpc node checksum failed". So every @rpc method scripts/server.gd
-## declares on its own root "Server" node has to be mirrored here too, even
-## the ones this proxy never actually needs to receive.
+## Client-side proxy node, attached at runtime as a Node named "Server" at the
+## SceneTree root by NetworkClient. Exists purely so Godot's RPC path-matching
+## finds a receiver for _receive_snapshot (the server calls it on its own root
+## "Server" node, so the client needs a node at /root/Server declaring the same
+## @rpc set). Not a real server.
 
 var owner_client: Node = null
 
 
-## Must match scripts/server.gd's own @rpc annotation on _receive_snapshot
-## exactly -- Godot enforces the receiving method's declared mode/transfer
-## config, a mismatch here would silently drop the call.
+## Must match scripts/server.gd's @rpc annotation on _receive_snapshot exactly.
 @rpc("any_peer", "call_remote", "reliable")
 func _receive_snapshot(snapshot_data: Dictionary) -> void:
 	if owner_client and is_instance_valid(owner_client):
 		owner_client._handle_snapshot(snapshot_data)
-
-
-## Mirrors scripts/server.gd's _kick -- never actually invoked on this side
-## (the server calls it on a Player_<id> node's own rpc_id target, not this
-## root proxy), declared only so the RPC checksum for this path matches.
-@rpc("any_peer", "call_remote", "reliable")
-func _kick(_reason: String) -> void:
-	pass
-
-
-## Mirrors scripts/server.gd's _disconnect -- same reasoning as _kick above.
-@rpc("call_remote", "reliable")
-func _disconnect(_reason: String) -> void:
-	pass