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/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-next.md §5.5.1) — the automated Mac passes gave
+# follow-up (MULTIPLAYER_SPEC.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-next.md §5.5 measures.
+	# 3v3 = 6 ships, matching the scenario MULTIPLAYER_SPEC.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