Fix: ArenaFieldSystem — restore the arena's clutter, cover and geysers (purge regression)

Operator at the A0 gate: "the world is very static." Part of that was a
regression from today's purge, and the audit's own signature pattern for the
third time: the system exists, the authoring is placed, and it silently
produces nothing.

Server/Economy/RoomFieldSystem.cs was deleted with the run/room FSM in
62e48a3b0. That was correct for the FSM, but the system was also the only
consumer of the ClutterFieldSpawner / CoverFieldSpawner / GeyserFieldSpawner
singletons. Those kept baking into Gameplay.unity and nothing read them:

  before:  Geyser 0   BlightClutter 0   (spawners configured for 14 + 2 + 3)
  after:   Geyser 2   BlightClutter 17

So the arena lost 14 destructible clutter pieces, 3 cover pieces and 2
periodically erupting geysers — a large share of everything that moved or
could be interacted with.

The old system could not be revived: it keyed every spawn off RunInfo,
RunRuntime, RoomPlan, RunMapMath, RegionMath and RoomTag teardown, all gone.
ArenaFieldSystem is a gym-scoped replacement that seeds ONCE at world start
and never tears down — Phase 1 is "no world, a gym", and everything
room-shaped belongs to the Phase-2 pocket generator, not here.

Contracts carried over deliberately, each load-bearing:
  - baked.WithPosition, never FromPosition (FromPosition resets Scale, a GhostField)
  - Geyser NextEruptTick born-correct off the live ServerTick via TickUtil.NonZero,
    staggered 60 ticks per instance so eruptions desync; 0 stays "not ready"
  - a 6 u keep-out ring around the arena origin (the player's landing spot)
  - clutter variant 3 = the explosive hazard, 0-2 inert dressing
  - a fixed arena seed: a gym wants the same arena every session

Verified live: geysers stamped 306 / 366 (staggered, Scale 1.00 preserved).

Navigation re-validated per the standing rule that adding Environment cover
can freeze movers, since enemies have NO pathfinding. First measurement showed
all four "frozen" — that was my error: they had already converged and were
attacking. Re-tested by moving the player across the arena: all four closed
23.5->20.4, 24.2->21.2, 23.9->20.9, 22.8->19.7 u through the cover field. No
soft-lock.

304/304 EditMode green.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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