Remove code with no live references, each confirmed dead via AST reference
analysis (no production callers and no importers), not just text search:
- chunk_belongs_to_document plus its dedicated tests and the now-orphaned
_insert_chunk test helper. The preview-target route already does a
single-query membership check and deliberately never called this helper.
- ingestion-progress.tsx and use-ingestion-events.ts (its only importer).
Superseded by the aggregate ingestion toast stack; zero importers.
- Unreferenced tests/fixtures/rag-preview sample files and their generator.
No behavior change. The only non-deletion edits reword two comments that
referenced the removed helper.
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