docs(multiplayer): split spec into MULTIPLAYER_SPEC.md, trim task doc to outstanding work

multiplayer-next.md was a 1662-line mix of standing architecture spec
and task-completion tracking, most of which was dense per-task DONE
evidence for finished Phases 0-6. Split it:

- MULTIPLAYER_SPEC.md (new): the locked architecture decisions, wire
  format, server-side input handling, prediction/reconciliation,
  latency/frame-rate budget, and match lifecycle state machine -
  standing design reference, not task-tracked.
- multiplayer-next.md (trimmed 1662 -> ~370 lines): only outstanding
  work remains - §0 status, §7 Phase 7/8 task tables condensed to
  "what's left" per task, §8-11 reference material (refactoring notes,
  gotchas, testing, flagged items). Phases 0-6 collapsed to a pointer
  at git history instead of ~500 lines of DONE evidence.

Also:
- Repointed every `multiplayer-next.md §N` code comment (N 1-6) across
  Game/scripts, Game/tools and Game/tests to MULTIPLAYER_SPEC.md, since
  those sections moved. Task-number references (`task N.N`, §7-11)
  correctly still point at multiplayer-next.md.
- Updated CLAUDE.md's doc index and docs/TECH_STACK.md's spec-section
  citations to match.
- TODO.md: added a "what's left to actually finish multiplayer
  (human-actionable)" checklist pulled from multiplayer-next.md §0 and
  docs/MATCHMAKING.md - things that need a person (hardware, a design
  decision, a Steam App ID, hands on a controller), not more agent code.

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-next.md). Pure functions only — no networking,
+# packets (§2 of MULTIPLAYER_SPEC.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-next.md for the ArenaBoundary/Ship/Ball
+# restated prose; see MULTIPLAYER_SPEC.md for the ArenaBoundary/Ship/Ball
 # constants these are sized against) ---
 const POS_RANGE := 64.0        # metres, ±
 const VEL_RANGE := 64.0        # m/s, ±