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/net_ship_predictor.gd

@@ -1,6 +1,6 @@
 extends RefCounted
 
-# Local-ship reconciliation policy (multiplayer-todo.md §4.4).  Kept out of
+# Local-ship reconciliation policy (multiplayer-next.md §4.4).  Kept out of
 # NetworkedMatch so the decision table is pure-testable; the imperative half
 # only writes Ship's existing Jolt-safe queued correction hooks.