Fix legacy site scene loading

[wass08]

Summary

• preserve embedded legacy site children until the existing scene migration flattens them
• prevent contentful legacy building subtrees from being pruned as unreachable
• cover the healer and full setScene path with regression tests

Test plan

• 760 core tests pass
• Editor monorepo build passes
• Production Casa model loads locally with 84/84 nodes
• Local bake route renders a 1920×1080 thumbnail with zero unreachable nodes

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Note

Low Risk
Narrow load-time change in scene graph healing for legacy site children; other child cleaning behavior is unchanged and covered by tests.

Overview
Fixes legacy scene loading where old site nodes stored full child objects in children instead of id strings. healSceneNodes used to treat those objects like invalid entries and drop them, which could disconnect or prune entire building subtrees before migration ran.

heal-scene-graph now keeps embedded objects on site nodes (still strips null and dedupes by child id) so the existing migrateNodes / setScene path can flatten them to string ids. Comments and strippedChildRefs wording are tightened to reflect “invalid” non-string refs only.

Regression coverage: heal-scene-graph.test for the healer behavior and use-scene-site-child-migration.test for a full embedded-site → flat graph load.

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