When qmd update runs against an index created before case-preservation,
documents may exist under lowercase paths (e.g. "skill.md" for a file
actually named "SKILL.md"). Add findOrMigrateLegacyDocument() that:
- Falls back to a lowercase lookup when the canonical path is not found
- Renames the document path in-place via UPDATE OR IGNORE
- Manually rebuilds the FTS entry (FTS5 INSERT OR REPLACE does not
reliably update existing rows via triggers)
- Handles UNIQUE conflicts gracefully (returns null on conflict)
Embeddings are keyed by content hash, so the rename preserves all
existing vectors — no re-embedding required.
Both the CLI indexer and the library reindexer share the same helper,
eliminating the duplication that a previous review flagged.
Includes integration tests for: successful migration, already-lowercase
no-op, and UNIQUE conflict handling.
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