editor: gate Load Build behind a verification dialog

[wass08]

What does this PR do?

Loading a JSON build previously called setScene blind and could crash the editor on schema-invalid input (e.g. items missing asset from a buggy IFC export). This PR gates Load Build behind a verification dialog that parses + validates the file first.

• New validateBuildJson in @pascal-app/core — pure pre-flight: structural errors (missing nodes / rootNodeIds, no valid roots, duplicate keys), per-node Zod parse with full SchemaIssue[] detail, orphan-parent / unknown-type / no-building / no-levels warnings, plus floor-area sum across slab polygons (minus holes).
• New LoadBuildDialog — title + ✓/✗, filename + size + node count, errors block (red), structure summary per node type (sites, buildings, levels, walls, doors, windows, items, slabs, ceilings, zones, scans) with floor area row, warnings block, collapsible Schema Details list grouped by node type (every failure shown — no truncation). Import is blocked on any hard error.
• Settings Panel handleFileLoad now parses → validates → opens the dialog. setScene is deferred to confirm.

Two schema bugs surfaced by the validator on real saves are fixed in the same PR:

• SiteNode.children is now z.array(z.string()) — every other node has used id arrays for years, and SiteRenderer / use-editor / site-panel all carried a typeof child === 'string' ? child : child.id ternary. The default-scene seed in use-scene.ts and the photo-to-scene MCP builder now pass building.id instead of the full object. migrateNodes flattens legacy nested-object children on load so older saved scenes still import.
• LevelNode.children now includes shelf — the editor allowed shelves parented to a level but the schema's union didn't list them, so a clean save → load roundtrip failed validation. The schema-vs-registry-as-source-of-truth discussion (drop the unions entirely and lean on relations.hosts) is captured in plans/editor-node-registry.md under Open questions in the private-editor repo.

How to test

1. bun dev:community (or the standalone editor) and open the build editor.
2. Open Settings → Save Build → save the current scene as JSON.
3. Settings → Load Build → pick the saved file. Dialog appears titled "Ready to import" with the structure summary + floor area. Click Replace current scene → scene swaps cleanly.
4. Load editor/packages/core/... or any IFC-converter output with bad nodes (items missing asset, doors with scalar position, etc.). Dialog title becomes "Cannot import this file", the error block lists the failure count, and the Schema Details section enumerates every failing node with nodeId · path — message. Import button is disabled.
5. Try loading an invalid JSON file (e.g. a .txt). Dialog opens with "File could not be parsed as JSON".
6. Load an empty object {}. Errors list both missing-nodes and missing-rootNodeIds.

Screenshots / screen recording

N/A here — happy to add a short clip if useful.

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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Project	Status	Preview	Updated (UTC)
pascal	🔴 Failed	–	May 20, 2026, 2:31 PM

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