[FEATURE] World features generate no collision — buildings, city volumes, and walls are solid to nothing

[Noisemaker111]

Problem

Everything the engine generates as world geometry is render-only. A game gets a building, a city, or a wall on screen and nothing in the world can collide with it.

Traced concretely for building():

building(...) → BuildingEnvironmentDescriptor → resolveStructureBuildings (packages/core/src/world/environmentSummary.ts:24) → GeneratedBuilding[] carrying a real bounds: Aabb (packages/core/src/world/buildings.ts:94-109). That result has exactly two destinations:

• render — packages/shell/src/environment/EnvironmentScene.tsx:314-336, which never touches collidersOf or scene.object.place
• populateNavGridFromEnvironment (packages/core/src/nav/navFromEnvironment.ts:14-23) — whose only callers are its own tests. No game builds a nav grid.

So a building() cluster is passable to the player and to NPCs. The player obstacle set is built solely from ctx.scene.object.inBox (packages/core/src/movement/playerMovement.ts:529), and generated structures never enter the object store.

Same for the rest:

Feature	Solid?
building() / structures	no
cityKind volumes (CityRenderer.tsx)	no — grep for collid returns nothing
world/walls.ts (wallSegments, roof plans)	no — pure geometry, emits no collider and no scene object
pads	no — only carves terrain height
vegetation	no
terrain()	floor only, never a wall

Only ctx.scene.object.place produces something solid.

Why it matters now

This is the reason Games/vice-isle/src/world.ts:91-95 abandoned structures: [building(...)] and re-authored every building as a placed scene object. That comment claims the clusters "blocked NPC navigation" — they did not; the nav bridge has never been called. They blocked nothing.

It also means the city kind can now render a fully kitted district (#1620) that no entity can touch — the better it looks, the worse the gap reads.

Acceptance criteria

• Generated world geometry can contribute collision without a game hand-re-authoring it as scene objects. A GeneratedBuilding already carries bounds; a city lot and a wall segment have footprints too.
• One notion of "solid": whatever the seam is, the player resolver and any AI obstacle consumer read the same source. Two parallel derivations is what produced this.
• Opt-in per feature — a decorative wall should still be able to be non-solid on purpose.
• A test asserting a given piece of world geometry is solid to the systems that claim to respect it. Nothing today would catch a divergence.
• Fix the stale comment at Games/vice-isle/src/world.ts:91-95 — it documents behaviour that never existed.

Related

• Walking NPCs consult no obstacles at all — filed separately; the two together are why "solid" currently means three different things.