Stage 21 harness: retryable transport failures, stricter nav metric

The Codex backend 429s under six concurrent harness processes, and the
per-cell catch wrote an error row that resume then treated as done -
baking a scored zero into the arm mean for a cell the model never got
to answer. Resume now keys on completed rows only, and since receipts
are append-only the retry appends a second row: the report keeps the
completed one, counts the error rows separately as an instrument
failure rate, and reports how many cells were de-duplicated.

Adds reached_by_read, derived from stored traces with no re-run.
reached_any counts library_find's context window either side of every
hit, which a couple of finds can blanket on a 10k document - it
saturates at 1.000 and stops discriminating. reached_by_read asks
whether a library_read, the model committing to a location, landed on
gold. On the partial asis pairing it separates the arms where
reached_any cannot.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H9ToHYJVDxfeJcJwdzrP2H