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/input_jitter_buffer.gd

@@ -1,7 +1,7 @@
 class_name InputJitterBuffer
 extends RefCounted
 
-# Per-player server-side input state (multiplayer-next.md §3, task 3.2).
+# Per-player server-side input state (MULTIPLAYER_SPEC.md §3; multiplayer-next.md 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-next.md §3.2's own numbers) — a duration, not a
+# 500ms at 60Hz (MULTIPLAYER_SPEC.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