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/video_settings.gd

@@ -24,7 +24,7 @@ extends Node
 # independently of stretch mode, since it scales the 3D viewport's own internal
 # resolution before this blit rather than the window itself. Task 0.15b also
 # found an unexplained ~6% non-uniform width scaling on this project's one
-# tested (Mac/Retina) machine — see multiplayer-next.md §5.5.1 — which needs
+# tested (Mac/Retina) machine — see MULTIPLAYER_SPEC.md §5.5.1 — which needs
 # understanding before stretch mode is touched, not blindly carrying into a
 # resolution-dependent change.
 #
@@ -49,7 +49,7 @@ const SETTINGS_PATH := "user://settings.cfg"
 # preset -> bundle applied to the individual fields below. CUSTOM has no
 # bundle: selecting it just stops future preset changes from overwriting
 # whatever the individual fields currently hold. Task 0.15b's measured
-# per-effect costs (multiplayer-next.md §5.5.1) were too noisy to rank these
+# per-effect costs (MULTIPLAYER_SPEC.md §5.5.1) were too noisy to rank these
 # against each other, so each rung is "meaningfully fewer full-screen passes
 # than the one above it" rather than a precisely tuned ladder.
 const PRESET_BUNDLES := {
@@ -252,7 +252,7 @@ func apply_fps_cap() -> void:
 # Called once by each arena's _ready() (and again on settings_changed, so an
 # already-loaded arena updates live) to fold the user's glow/brightness
 # preference into that arena's own baked Environment tuning, and to gate the
-# preset-controlled full-screen passes (§5.5 of multiplayer-next.md).
+# preset-controlled full-screen passes (§5.5 of MULTIPLAYER_SPEC.md).
 func apply_to_environment(env: Environment) -> void:
 	if env == null:
 		return