feat(staging): baked stage-pool glow (deferred from #224 / E1.3)

[bradleypmartin]

Deferred follow-up split out of #224 (E1.3 environment surround) under the v1.0 staging epic #221.

Context

PR #245 (#224) shipped the environment as a near-black flat backdrop + a per-scene, exhibit-owned ContrastPit (src/scaffold/staging/ContrastPit.ts) sized to each scene's floor cutout. During #224 a "baked stage-pool glow" (a soft radial light-pool reading as an overhead spotlight on the stage) was scoped out:

• Roundtable + smoke surfaced that a shell-owned pool plane structurally conflicts with the per-exhibit floors (differing outerHalfExtent / backExtension, occlusion / z-fight, invisibility under an opaque floor) — the same class of conflict that drove ContrastPit to per-scene ownership.
• It needs a per-scene-floor-aware design and reads as closer to E1.4 control-plinth / stage-lighting work than to backdrop atmosphere.
• v1.0.md §2 ranks E1.3 atmosphere as the first thing to cut; the pool-glow was the cuttable richness.

Scope

A baked (no dynamic lights — Quest 3S fragment budget) soft ground-glow under the focal volume suggesting an overhead stage spotlight, designed per-scene consistent with the now-established exhibit-owned staging pattern (StageFloor / StageRailing / StageInnerRailing / ContrastPit). Likely sized off the published StageFloor extents like the railing/pit already are.

Out of scope / notes

• No dynamic or movable lights; baked texture / vertex approach only.
• Compose with ContrastPit (the pool sits above the floor; the pit below it) without z-fighting.
• Decide whether it becomes a formal child of feat(staging): production-quality environmental presence (v1.0 epic) #221 or stays a standalone polish item.

Refs: #224, #245, #221; _private/plans/224-environment-surround.md §1 / §8 (maintainer-only).

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