feat(doors): bake open-animation clips for sliding/garage/folding/pocket/barn doors

Only swing doors (hinged/double/french) baked an open clip into the GLB —
they carry a `pascalSwingLeaf` marker the exporter reads. Every operation
door type (sliding, pocket, barn, folding, garage-sectional/rollup/tiltup)
baked its `operationState` straight into mesh vertex positions at build
time, so the exporter had no re-poseable node to sample and the artifact
never flagged them `openable`.

Give operation doors the same build-once + pose-at-t split windows already
use. Each builder now emits its moving parts in a named group at the CLOSED
pose, and `poseDoorMovingParts` (the single source of truth, shared by the
live system and the GLB exporter) drives the open motion:

- sliding/pocket/barn: rigid leaf translation
- garage-tiltup: rigid hinge about the lintel
- folding: hinged accordion chain (nested groups, per-joint fold)
- garage-sectional: per-panel groups posed along the overhead curve
- garage-rollup: the one type whose live geometry changes (slats roll onto
  a drum, which a glTF clip can't express) keeps its full-detail live
  rebuild; the curtain is wrapped in a top-pivoted group the exporter
  scales up into the lintel as the baked approximation.

The exporter samples each operation door's motion into keyframe tracks
(16 segments) so the non-linear rigs (curve, accordion) stay faithful, and
stamps `extras.openable` + `extras.clips` so any glTF consumer can play it.

Tests: per-type kinematics (groups build, rest closed, open) +
sliding/roll-up clip baking (sampled position/scale tracks).

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

packages/viewer/src/index.ts

@@ -111,7 +111,7 @@ export {
   createColumnTorusGeometry,
 } from './systems/column/column-geometry'
 export { DoorAnimationSystem } from './systems/door/door-animation-system'
-export { buildDoorPreviewMesh, DoorSystem } from './systems/door/door-system'
+export { buildDoorPreviewMesh, DoorSystem, poseDoorMovingParts } from './systems/door/door-system'
 export { ElevatorInteractionSystem } from './systems/elevator/elevator-interaction-system'
 // Fence system follows the wall re-export pattern — composed into the
 // registry-driven fence definition's `def.system`. Removed in Phase 6