fix(viewer): stack levels per building

[Aymericr]

Fixes #511.

What changed

• compute cumulative level heights independently for each building
• share the same stacking calculation between the live LevelSystem and the temporary true-position snap used by exports and thumbnails
• preserve the existing single stack for legacy orphan levels
• cover single-building, multi-building, and orphan-level cases with focused tests

Why

LevelSystem previously sorted every registered level together and used one global cumulative Y offset. A second building's ground floor therefore started above the first building instead of on its own ground plane. The export/thumbnail helper duplicated the same global-stack behavior.

Thanks @archicorp46-debug for the precise report and reproduction.

Verification

• bun test packages/viewer/src/systems/level/level-stacking.test.ts
• bun run --filter @pascal-app/viewer build
• bun check
• bun check-types

---

Note

Medium Risk
Changes core 3D level positioning for all scenes; behavior is well-tested for multi-building and legacy orphans but wrong building resolution could mis-stack levels visually.

Overview
Fixes multi-building scenes where every level was stacked on one global Y ladder, so a second building’s ground floor appeared above the first.

The PR adds level-stacking helpers: getLevelBuildingId (building parent or legacy children lookup) and getLevelStackPositions (cumulative height per buildingId, with orphan levels sharing one null stack). LevelSystem and snapLevelsToTruePositions in level-utils now build the same entries and read Y from that map instead of a single walk over all levels. Bun tests cover single-building order, independent ground planes per building, and orphan legacy stacking.

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