Carry collections through Save Build / Load Build

[chiliec]

What does this PR do?

Fixes #734 — Save Build / Load Build dropped collections on both sides. Same shape as #729 (materials):

• handleSaveBuild now includes collections in the saved sceneData.
• validateBuildJson gets a collections pass: each entry is checked against the Collection shape (id, name, nodeIds: string[], optional color/controlNodeId); invalid entries are skipped with an invalid_collections warning that names the skipped ids, a non-object collections is ignored with a warning. Valid entries land on parsed.collections.
• handleConfirmImport passes parsed.collections through to setScene (which already accepted extra.collections).

How to test

1. Create a collection in the editor, Save Build, then Load Build the same file → the collection is still there (before: gone).
2. bun --cwd packages/core run test — 4 new tests in validate-build-json.test.ts under collections (carry-through, per-entry skip with warning, non-object warning, absent → omitted from parsed).
3. Reverting the validate-build-json.ts change makes 3 of those tests fail; with it they pass. Full packages/core suite: 1430 pass / 0 fail. packages/editor tests: 836 pass / 0 fail, tsc --noEmit for packages/editor clean.

Screenshots / screen recording

n/a — no UI change, only the saved file contents and the import pass.

Checklist

• I've tested this locally with bun dev (verified via the package test suites + tsc --noEmit instead; happy to add a clip if you'd like one)
• 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

Low Risk
Scoped to build JSON validation and settings-panel save/import wiring; no auth or security paths, and invalid collection data is warn-and-skip rather than hard-failing import.

Overview
Fixes collections being dropped on Save Build and Load Build, mirroring the existing materials round-trip.

Save Build now writes collections from the scene store into the exported JSON alongside nodes, materials, and plugins. Load Build passes parsed.collections into setScene after import validation.

validateBuildJson gains a collections pass: valid entries (required id, name, nodeIds: string[], optional color / controlNodeId) appear on parsed.collections; bad entries are skipped with an invalid_collections warning that lists skipped ids; a non-object collections field is ignored with a warning; absent collections stay omitted from parsed. Four unit tests cover carry-through, partial skip, invalid top-level shape, and omission when missing.

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[pascal]

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Error id: b2126f81-7fc5-4800-9c2c-d21556b11d63