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/tests/cases/test_net_codec.gd

@@ -97,7 +97,7 @@ func test_snapshot_roundtrip_seven_bodies() -> void:
 
 	var packet := NetCodec.pack_snapshot(555, -2, 1234, segment)
 	assert_eq(packet.size(), NetCodec.SNAPSHOT_CLIENT_HEADER_SIZE + segment.size(), "full packet size")
-	assert_eq(packet.size(), 169, "matches multiplayer-todo.md §2.4's 169 B payload figure for 7 bodies")
+	assert_eq(packet.size(), 169, "matches multiplayer-next.md §2.4's 169 B payload figure for 7 bodies")
 
 	var decoded := NetCodec.unpack_snapshot(packet)
 	assert_eq(decoded["last_input_seq"], 555, "last_input_seq")

Game/tests/lobby_smoke.gd

@@ -8,7 +8,7 @@ extends Node
 # get_tree().current_scene, and calling it from a node that ISN'T an
 # ancestor-chain match for current_scene (as an earlier draft of this test
 # did, by add_child()-ing lobby.tscn under this driver) hung completely
-# on disconnect — see multiplayer-todo.md §9 gotcha 27.
+# on disconnect — see multiplayer-next.md §9 gotcha 27.
 #
 # The host role loading lobby.tscn is deliberate, not an oversight: a
 # *dedicated* server (server_boot.tscn) never loads it, but a self-hosting

Game/tests/match_net_smoke.gd

@@ -97,7 +97,7 @@ func _on_player_joined(peer_id: int, player_name: String) -> void:
 	_finish(true, "host saw player_joined (peer_id=%d, name=%s)" % [peer_id, player_name])
 
 
-# Adversarial-review regression (multiplayer-todo.md §9): MatchNet.roster
+# Adversarial-review regression (multiplayer-next.md §9): MatchNet.roster
 # used to have no path that cleared it when a HOST itself called
 # NetworkManager.shutdown() — only the client-side disconnect signal did.
 # Host -> client joins -> host leaves (shutdown) -> host again used to