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/sim_constants.gd

@@ -4,7 +4,7 @@ class_name SimConstants
 # constant derived from "60 Hz" (Ship._tick_scaled's decay reference,
 # reaction_ticks' export range, TrainingMode.TICKS_PER_SIM_SECOND) reads this
 # instead of restating the literal, so changing it changes every derived
-# constant coherently — see multiplayer-next.md §5.6 on why a future 120 Hz
+# constant coherently — see MULTIPLAYER_SPEC.md §5.6 on why a future 120 Hz
 # simulation needs to be a config change plus a retrain, not a protocol
 # rewrite hunting down bare 60s.
 #