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/tools/gpu_profile_harness.gd

@@ -1,7 +1,7 @@
 extends Node
 
 # One-off GPU frame-time profiling harness for task 0.15b's real-hardware
-# follow-up (multiplayer-todo.md §5.5.1) — the automated Mac passes gave
+# follow-up (multiplayer-next.md §5.5.1) — the automated Mac passes gave
 # inconsistent, sometimes implausible numbers (stale-process contention,
 # and Apple Silicon's tile-based GPU architecture is a poor stand-in for the
 # target reference hardware). Run this directly on a machine with a real
@@ -47,7 +47,7 @@ func _ready() -> void:
 
 	var match_scene := load("res://scenes/match.tscn") as PackedScene
 	_match = match_scene.instantiate()
-	# 3v3 = 6 ships, matching the scenario multiplayer-todo.md §5.5 measures.
+	# 3v3 = 6 ships, matching the scenario multiplayer-next.md §5.5 measures.
 	_match.team_size = 3
 	# Direct-scene-run fallback path (see match_mode.gd:_make_opponent_controller)
 	# — gives every AI ship a real trained policy so thruster VFX/movement