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/perf_overlay.gd

@@ -5,7 +5,7 @@ extends CanvasLayer
 # Performance monitors; never touches rendering or gameplay state. Exists so
 # 0.17/0.17b's graphics presets and resolution scaling are self-diagnosing —
 # TIME_PROCESS vs total frame time tells the player whether they're CPU- or
-# GPU-bound. See multiplayer-todo.md task 0.20.
+# GPU-bound. See multiplayer-next.md task 0.20.
 
 # ~2s of history at 60 fps; enough to make p50/p99 meaningful without the
 # history itself being a rate-dependent quantity.