docs(multiplayer): consolidate tracking into one document

multiplayer-todo.md and multiplayer-next.md tracked overlapping
information in two places. Fold everything into multiplayer-next.md
(architecture decisions, wire format, task breakdown with checkboxes,
gotchas list, testing notes) and delete multiplayer-todo.md. Section
numbers are unchanged, so existing code comments citing them by
section/task number still resolve; update every such reference to
point at the new filename.

Game/scripts/network_manager.gd

@@ -3,7 +3,7 @@ extends Node
 # Autoload (project.godot [autoload] NetworkManager). Owns transport-neutral
 # hosting, joining, shutdown, and connection-state signals. Lives
 # at a fixed autoload path so RPC NodePaths never depend on which scene is
-# loaded (§1.3 of multiplayer-todo.md's derived decisions).
+# loaded (§1.3 of multiplayer-next.md's derived decisions).
 #
 # server_relay = false is set the moment a peer exists: the default `true`
 # lets any client rpc() any other client *through the server*, which this
@@ -25,7 +25,7 @@ extends Node
 # pays the same tax again. set_multiplayer_poll_enabled(false) below turns
 # that off; every caller that sends or expects to receive on a tight cadence
 # must now call NetworkManager.poll() itself. The intended placement per
-# multiplayer-todo.md §7 task 1.3 (client: end of _physics_process after
+# multiplayer-next.md §7 task 1.3 (client: end of _physics_process after
 # sending input, plus top of both _process and _physics_process for receive;
 # server: tick start to drain, tick end to flush) has no real per-tick caller
 # yet — that lands with the input/snapshot pipeline (tasks 1.4+, Phase 2-3).