Art(engine): ArtStaging underwater ambiance + hero triad baked in

Underwater staging: undulating seabed mesh (no platform edge), caustics
cookie spot, marine-snow particles, teal depth fog, and 'light is territory'
pool lighting (warm beacon + cold bioluminescent pools, deep dark between).
Baked posed statics into the slots: SM_Suit_Bathynaut + SM_Creature_Drowner
(glTFast) alongside the crate + flora clutter + rocks. Completes the A0
style-proof triad (prop+creature+suit) in-engine under gameplay conditions.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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