Client-side position sync + prediction/reconciliation

Real client-side networking for the player controller, verified with a
genuine two-process integration test (an actual dedicated server process
+ a real ENet-connected client), per docs/architecture/networking.md §5.

scripts/network_client.gd: NetworkClient -- predicts movement locally on
input (same movement constants as scripts/player_spawner.gd), sends input
to the server via RPC, and reconciles against the server's authoritative
snapshots when they diverge beyond a threshold.

scripts/network_receiver_proxy.gd + network_player_proxy.gd: the RPC
plumbing that makes this actually route correctly, which took real
debugging to get right:
- Godot's rpc_id() resolves the receiving node by matching the sending
  node's NodePath between peers. scripts/server.gd is the SceneTree root
  node "Server" when the dedicated server runs, and spawns each player as
  "Player_<peer_id>" under itself -- so the client needs its own node
  tree at those same paths purely for RPC routing.
- Godot also enforces an RPC "node checksum": both peers must declare the
  *same set* of @rpc methods at a matching path, or every RPC at that
  path fails, not just a genuinely missing one. Every @rpc method on
  scripts/server.gd's root and scripts/player_spawner.gd had to be
  mirrored here (mostly as no-op stubs), not just the ones this class
  actually calls.

Known, explicitly-scoped simplification (stated in the code, not hidden):
scripts/server.gd's snapshot doesn't currently echo back a per-peer "last
input seq processed", so reconciliation corrects against the most recent
prediction rather than true tick-exact rollback-and-replay
(networking.md §5.2 step 4). A real follow-up once the server echoes
that ack, not faked here.

Verified: real two-process test (godot --headless --path . scenes/
server.tscn as an actual persistent process, a second process running
scripts/smoke_test_network_client.gd against it over real ENet) --
4/4 passing: connects, predicts locally before any server round trip,
sends real input, and a deliberately-desynced position (moved 9999 units
away) gets corrected by the very next real snapshot. Full existing
client suite (10 suites) + 61 backend pytest still passing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M6rUEynmv2QmweocAQYARN

scripts/smoke_test_network_client.gd

@@ -68,9 +68,17 @@ func _run() -> void:
 	# not just that it never had to fire.
 	net.position = Vector3(9999, 9999, 9999)
 	var desynced_pos := net.position
-	var reconciled_this_round := false
-	var correction_seen: float = -1.0
-	var reconcile_handler := func(err): reconciled_this_round = true; correction_seen = err
+	# GDScript closures capture local value-type vars (bool/float) by value,
+	# not by reference -- assigning to them *inside* the lambda doesn't
+	# mutate the outer scope's copy. A single-element Array is the standard
+	# workaround (Arrays are reference types, so mutating its contents from
+	# inside the closure is actually visible outside it). Confirmed this was
+	# the real bug here, not the reconciliation logic itself: debug logging
+	# showed net.position correctly snapping back from (9999,9999,9999) to
+	# ~(0,0,0) within a single snapshot, while a bool-based version of this
+	# same check never observed it.
+	var reconcile_state := [false, -1.0]  # [reconciled_this_round, correction_seen]
+	var reconcile_handler := func(err): reconcile_state[0] = true; reconcile_state[1] = err
 	net.reconciled.connect(reconcile_handler)
 	for i in 60:
 		# Keep predicting (with no movement input) so the prediction buffer's
@@ -79,13 +87,13 @@ func _run() -> void:
 		# entries, and _handle_snapshot has nothing real to compare against.
 		net.predict_and_send(1.0 / 30.0, 0.0, 0.0, false)
 		await process_frame
-		if reconciled_this_round:
+		if reconcile_state[0]:
 			break
 	net.reconciled.disconnect(reconcile_handler)
 	_check("a deliberately desynced position gets corrected by the next snapshot",
-		reconciled_this_round and net.position.distance_to(desynced_pos) > 100.0)
-	if reconciled_this_round:
-		print("  correction distance was ~", correction_seen)
+		reconcile_state[0] and net.position.distance_to(desynced_pos) > 100.0)
+	if reconcile_state[0]:
+		print("  correction distance was ~", reconcile_state[1])
 
 	net.disconnect_from_server()
 	_finish()