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_codec.gd

@@ -1,7 +1,7 @@
 class_name NetCodec
 
 # Wire-format constants, quantisers, and pack/unpack for the two hot-path
-# packets (§2 of multiplayer-todo.md). Pure functions only — no networking,
+# packets (§2 of multiplayer-next.md). Pure functions only — no networking,
 # no autoload state — so they're testable head-on by tests/test_runner.tscn
 # without a live connection.
 #
@@ -48,7 +48,7 @@ const BODY_FLAG_STALLED := 1 << 5
 const BODY_FLAG_QUAT_W_SIGN := 1 << 6
 
 # --- Quantisation ranges (§2.4 — derived from arena/gameplay constants, not
-# restated prose; see multiplayer-todo.md for the ArenaBoundary/Ship/Ball
+# restated prose; see multiplayer-next.md for the ArenaBoundary/Ship/Ball
 # constants these are sized against) ---
 const POS_RANGE := 64.0        # metres, ±
 const VEL_RANGE := 64.0        # m/s, ±