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.

CLAUDE.md

@@ -14,7 +14,8 @@ Because the gameplay concept (vehicle soccer) can't be copyrighted but specific
 
 The prose docs carry far more design rationale than the code comments, and several are load-bearing:
 
-- `multiplayer-next.md` — **the single multiplayer tracking document**: architecture decisions, the wire format, implementation evidence, a numbered "gotchas" list (§9), and the current task breakdown with checkboxes, all in one file. Start at §0 for "what's left". Code comments cite it constantly by section/task number (`§2.4`, `task 5.10`); when a comment does, that section is the real explanation. Phases 0–6 are done and mostly archival; day-to-day work is Phase 7 (Steam) and Phase 8 (matchmaking), whose numbered task breakdown and acceptance criteria live in §7, because that is the format tasks are picked up from.
+- `multiplayer-next.md` — **the multiplayer task-tracking document**: outstanding work (§0), a numbered "gotchas" list (§9), and the current task breakdown with checkboxes (§7), all in one file. Start at §0 for "what's left". Day-to-day work is Phase 7 (Steam) and Phase 8 (matchmaking), whose numbered task breakdown and acceptance criteria live in §7, because that is the format tasks are picked up from. Phases 0–6 are done and archival — their evidence lives in git history, not the current doc.
+- `MULTIPLAYER_SPEC.md` — the architecture decisions, wire format, server-side input handling, prediction/reconciliation, latency/frame-rate budget and match lifecycle state machine, as sections 1–6. Code comments across `Game/scripts/` cite it constantly by section number (`§2.4`, `§4.1`); many still say `multiplayer-next.md §N` for `N` 1–6 from before this doc was split out — when a comment does, the content is now here, not there. `multiplayer-next.md`'s own §7+ cites `§N` the same way and disambiguates by number (1–6 → this doc, 7+ → itself).
 - `TRAINING.md` — the full RL workflow (training, curriculum generations, export, eval, difficulty tiers).
 - `SERVER.md` — dedicated-server build, config, systemd deploy, sizing.
 - `STEAM.md` — optional GodotSteam custom-build setup and the transport contract.