feat(shell): shared cluster-wide "scene plinth" hosting the SceneRack

[bradleypmartin]

Surfaced from #225 PR1 (#250 / #254) headset smoke (Brad, 2026-05-22).

"might even need ANOTHER plinth aside from the one we've made and the #226 plan: the scene selector is a little janky-looking now just floating up in the air. THIS theoretical 'scene plinth' should probably be shared among all the exhibits in a common world location." — Brad

The SceneRack (src/shell/SceneRack.ts) is shell-owned cluster navigation — one row of SceneTab bubbles at the same world position across every cluster member. After PR1 (#250) it tracks the control-plinth z (SCENE_RACK_Z = 0.05) so the bubbles aren't stranded over the cutout, but they still float in mid-air with no surface to anchor against.

A third plinth, not the existing two

The cluster is heading toward a trio of plinth types:

Plinth	Scope	Source
Control plinth	per-exhibit knobs/dials/readouts	#225 (delivered v0 in #250)
Info plinth	per-exhibit in-world instructional reader	#226
Scene plinth (this issue)	cluster-wide navigation surface — one shared world location	this issue

The defining feature versus #225 / #226 is shell-owned, single instance, shared across every cluster scene — never re-allocated, never per-exhibit. That ownership shape is the same as SceneRack itself today; the visual upgrade is adding the surface beneath it.

Direction (open)

• Footprint and silhouette — probably distinct from the drafting-table-console silhouette of the control plinth, since the role is different (navigation between scenes vs. operating one scene's controls). Could read more as a museum kiosk / lectern / vitrine.
• Worldspace placement — needs to compose with the cluster's existing staging (floor, railing, contrast pit) without competing for the user's attention against the math object. The current SceneRack anchor at (-0.44, 1.73, 0.05) is the starting point; the plinth lifts the bulbs off of "floating in air" toward "this is the scene-selector station."
• Ownership — shell-owned, allocated once at boot in shell.ts alongside the SceneRack itself, never per-exhibit. Lifetime is the shell lifetime, not the exhibit lifetime. This is the cleanest break from feat(staging): control plinth hosting interactive UI (sub-epic) #225 (exhibit-owned, allocated in mount()).
• Composition with the cluster's other staging — the scene plinth is the FIRST shell-owned worldspace plinth, so it sets the precedent for the v1.0.md §4 "persistent worldspace plinths default to shell-owned" rule that the control plinth (feat(staging): control plinth hosting interactive UI (sub-epic) #225) deliberately broke. Worth a callout in v1.0.md when this lands.

Open questions for plan-doc / roundtable

• Does the scene plinth need its own primitive in scaffold/staging/, or can it reuse the createPlinth(...) factory from feat(staging): control plinth hosting interactive UI (sub-epic) #225 with different ownership semantics + dimensions?
• Does it host anything beyond the SceneRack — e.g., the FPS overlay, a session-level "back to home" affordance, or future cluster-level metadata?
• Where does it live in pancake vs. VR? (Same per-mode tension as the control plinth has under feat(staging): pancake-vs-VR plinth calibration sweep (#225 E1.4e) #253, but resolved once at shell-boot instead of per-scene.)

Code anchors

• SceneRack today: src/shell/SceneRack.ts — anchor constants at file top, currently SCENE_RACK_Y = 1.73, SCENE_RACK_CENTER_X = -0.44, SCENE_RACK_Z = 0.05.
• Allocation site: src/shell/shell.ts step 5 (#150) — const rack = new SceneRack({...}); scene.add(rack.group);. The "scene plinth" would slot in alongside it.
• Cluster context: src/shell/clusters.ts (cluster identity), src/shell/Exhibit.ts (cluster-membership filter).

Refs

• Surfaced by: feat(staging): plinth primitive + slot-mount API + quadrics on plinth (#225 E1.4a/b) #250 / feat(staging): plinth primitive + slot-mount API + quadrics on plinth (#250) #254 (post-PR1 visual smoke).
• Sibling plinth issues: feat(staging): control plinth hosting interactive UI (sub-epic) #225 (control plinth, exhibit-owned, delivered v0), feat(staging): info plinth — in-world instructional reader (sub-epic) #226 (info plinth, exhibit-owned in-world reader). This issue completes the trio with shell-owned cluster-wide navigation.
• Parent epic: feat(staging): production-quality environmental presence (v1.0 epic) #221 (v1.0 staged-exhibit vocabulary).
• v1.0 plan: _private/plans/v1.0.md §4 (persistent-worldspace-plinth ownership rule — the scene plinth would be the rule's first confirming instance after feat(staging): control plinth hosting interactive UI (sub-epic) #225's deliberate break).

🤖 Generated with Claude Code