core: repair null parent links so stored scenes pass authority validation

[Aymericr]

What does this PR do?

Repairs the stored-scene corruption behind the hosted "Live collaboration is unavailable for this project. Editing is paused to protect the scene." incident (Sentry MONOREPO-EDITOR-JR/JQ/GF): scenes whose children arrays link site → building → level while building/level carry parentId: null. The editor renders these scenes by traversing children, but the hosted scene authority validates parent/child symmetry and rejected every snapshot, locking sessions read-only and silently discarding tool placements (ghost + SFX with no commit).

• healSceneNodes gains a repair pass: a node with a null parentId that exactly one parent claims via children points back at that parent. Embedded legacy site children claim by id, so the flattened flat-map node is repaired too. Ambiguous multi-claim nodes are left alone.
• loadScene writes explicit parent links for the default Site → Building → Level hierarchy (it previously relied on the schema's parentId: null default — one of the two writers of the corrupt shape; the other was the legacy embedded-site-child flatten, which pass 3 now covers on load).
• New @pascal-app/core/scene-migrations barrel exports healSceneNodes alongside removeRetiredDrawingSheetNodes, keeping the entry server-safe so the hosted authority can apply the identical normalization before validating.

How to test

1. bun test packages/core/src/utils/heal-scene-graph.test.ts — new cases cover the null-parent chain repair, embedded legacy site-child claims, ambiguous multi-claim nodes, and untouched dangling parents.
2. bun test packages/core/src/store/use-scene-load-scene.test.ts — the default scene now carries symmetric parent links.
3. bun test packages/core/src — full core suite (930 tests).
4. In the hosted app, feed a stored graph with building/level at parentId: null through normalizeAuthoritySceneGraph (companion private-editor change) and confirm apiGraphSchema accepts it.

Screenshots / screen recording

N/A — non-visual data-repair change.

Checklist

• I've tested this locally with bun dev
• My code follows the existing code style (run bun check to verify)
• I've updated relevant documentation (if applicable)
• This PR targets the main branch

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Note

Medium Risk
Changes scene graph normalization used on load and by the hosted authority; incorrect repair logic could alter hierarchy, though guards limit repairs to unambiguous single-claim cases.

Overview
Fixes stored scenes where children links site → building → level but parentId stays null on building/level, which blocked hosted scene authority validation and paused live collaboration.

healSceneNodes adds a third pass: if a node has parentId: null and exactly one parent lists it in children (including embedded legacy site children), it sets parentId to that parent; ambiguous multi-parent claims are unchanged.

loadScene now sets explicit parentId on the default Site → Building → Level graph instead of relying on schema defaults, so new default scenes save with symmetric parent/child links.

The @pascal-app/core/scene-migrations export is pointed at a new server-safe barrel that re-exports healSceneNodes and removeRetiredDrawingSheetNodes for shared client/authority normalization.

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