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/network_receiver_proxy.gd

@@ -20,7 +20,6 @@ var owner_client: Node = null
 ## config, a mismatch here would silently drop the call.
 @rpc("any_peer", "call_remote", "reliable")
 func _receive_snapshot(snapshot_data: Dictionary) -> void:
-	print("DEBUG proxy received snapshot: ", snapshot_data)
 	if owner_client and is_instance_valid(owner_client):
 		owner_client._handle_snapshot(snapshot_data)