LANTERN realignment P3: one look everywhere + scene surgery

- PostFX_Lantern.asset: the ONE shared grade (ACES, bloom 0.6 cool-tinted for the
  HDR gloam, vignette 0.28, faint teal filter) applied to Game, DevSandbox, and
  ArtStaging; PostFX_DarkSciFi / PostFX_Daylight / Sky_DaytimeProcedural deleted.
- Env_SeabedKit.prefab: ArtStaging's environment (seabed, flora, rocks, marine
  snow, caustics, warm pool, gloam fill, key light, StagingAmbiance animator)
  prefab-ized and placed in Game + DevSandbox; ArtStaging connected to it.
- Scene surgery: old DevSandbox.unity deleted; Gym.unity RENAMED DevSandbox.unity
  (GUID preserved -> GymSub wiring + the F1/F2 dev scripts that gate on the
  'DevSandbox' name come back to life). SyntyWorld root deleted from DevSandbox;
  BaseBiome/ExpeditionBiome/Slot1 deleted from Game.
- RenderSettings unified: no skybox, Exp2 teal fog {0.02,0.10,0.12} @ 0.035, flat
  near-black ambient {0.03,0.055,0.08}; camera clearFlags -> solid deep-water.
- ScenePolicy.IsGameplayScene() replaces the six scene.name=="Game" string gates
  (Game + DevSandbox share the dynamic look; menu/ArtStaging untouched).
- WorldAtmosphereSystem rewritten as water-column murk (LANTERN defaults; biome
  variants re-meant to kelp/trench/gloam-bloom; Ground_Arid tint block deleted).

390 green; Play-verified in Game: murk values live, warm-vs-cold reads, no errors.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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