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/input_jitter_buffer.gd

@@ -1,7 +1,7 @@
 class_name InputJitterBuffer
 extends RefCounted
 
-# Per-player server-side input state (multiplayer-todo.md §3, task 3.2).
+# Per-player server-side input state (multiplayer-next.md §3, task 3.2).
 # Deliberately a standalone RefCounted with no scene/RPC dependency — same
 # reason net_codec.gd and net_interpolator.gd are pure classes — so task
 # 3.5's unit tests can drive it with scripted arrival traces with no live
@@ -18,7 +18,7 @@ extends RefCounted
 # class's, since only the caller knows the current server tick.
 
 const RING_SIZE := 32
-# 500ms at 60Hz (multiplayer-todo.md §3.2's own numbers) — a duration, not a
+# 500ms at 60Hz (multiplayer-next.md §3.2's own numbers) — a duration, not a
 # tick-rate-derived constant, so left as a literal rather than pulling in
 # SimConstants for one number.
 const STARVE_ZERO_TICKS := 30