What does this PR do?

Three small editor-side polish items (split into one commit each):

1. feat(sfx) — paint-apply sound. Registers a paintApply SFX (rate/volume jitter + 60 ms min interval so rapid multi-face painting doesn't machine-gun), wires the sfx:paint-apply bus event, and emits it from the material-paint click chokepoint in the selection manager (fires on apply, not hover/preview). Adds apps/editor/public/audios/sfx/paint_apply.mp3.
2. fix(item) — duplicated items keep their painted slot materials. Duplicating an item drops into the catalog placement flow, which rebuilt the draft from asset + transform and never carried node.slots, so the copy lost every painted slot. Threads slots through the draft path: useDraftNode.create seeds it, commit() forwards it to the final node, the placement coordinator passes it to its lazy wall/ceiling draft creates, and the item move tool supplies node.slots for both the floor and wall/ceiling duplicate paths. (Other kinds clone via structuredClone + parse + createNode, which already preserved slots — this was item-specific.)
3. docs(review-skill) — slots + world-scale-UV convention. Adds a Section B rule to review-architecture: a new node kind / geometry change exposing paintable parts must carry overrides in a schema slots record resolved via def.capabilities.paint (and thread it through clone/duplicate/preset paths), and texturable def.geometry must emit UVs in metres (1 unit = 1 m) so catalog finishes tile. Surfaced in the skill description.

How to test

1. SFX: enter paint mode, paint a surface → the paint cue plays; rapid painting stays subtle (gap + jitter).
2. Duplicate slots: paint an item's slot a catalog material → duplicate the item → the copy keeps the material (test a floor item and a wall-hosted item).
3. Review skill: skim Section B of .agents/skills/review-architecture/SKILL.md and the new convention bullets.

Screenshots / screen recording

N/A — audio + behavior + docs.

Checklist

• I've tested this locally with bun dev — static checks only (typecheck + biome clean); runtime SFX/duplicate behavior validated locally by the author
• My code follows the existing code style (biome check clean)
• I've updated relevant documentation (review-architecture skill)
• This PR targets the main branch