Memoizes an expensive bytes→value parse keyed by the file's metadata
fingerprint (eTag, else lastModified+sizeBytes): an unchanged file costs
one getMetadata round-trip instead of a full read+parse. Fails open when
metadata is unavailable, never caches parse failures, dedupes concurrent
parses, and keeps one entry per FileRef. Documented in
docs/llm-document-source.md (freshness section).

Motivation: fugue-dags re-parses five workbooks (25k-row master list)
per request — ~7-9s of every call, dwarfing the LLM.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>