Bi-temporal claims: record when a fact was true, not just when it was learned

Git gives every memory system time for free — *when we recorded it*. It does not give *when it was true*. Separating event time from record time is what turns "what did the file say on this date" into "what did we believe was true as of this date" — the question an auditor actually asks. Design sketch: optional `valid_from`/`valid_to` on records or claims, additive frontmatter, ranker awareness. Open for design discussion; implementation follows the discussion, not the other way around.

No files or tests are named. Start by reviewing the existing records or claims model, frontmatter handling, and ranker behavior, then use the issue's design discussion to define how optional valid_from and valid_to should represent event time separately from record time. Done means an agreed design exists before implementation begins.

Written by the indexing model from the issue text.