Batch rechecks, and say which flip makes a Score decisive #1253

This was generated by AI during triage. Rewritten from the grill-with-docs session in docs/grill-with-docs/1252-1253-bar-and-rechecks.md (Q3.3). The original problem statement is in this issue's edit history.

Parent

#1252

What to build

Keep today's rule for a decisive Score: a Score is decisive when any one flip of its pass or its wrong finding would change the provisional Verdict. Borderline Scores are still always rechecked. Under #1252's two-level Bar, a pass with room has few decisive Scores, so rechecks gather where a Score really decides the Verdict. Make those rechecks explainable and cheap to record.

• Which flip. In thirdshift experiments <id> --json, each entry in pending_rechecks says which flip makes it decisive (its pass, its wrong finding, or both) and the provisional Verdict that flip gives.
• A batch recheck. One command lists every pending recheck, with its suite, Eval task, Arm, Trial and the reason it's pending, plus a fingerprint of that list. A second form takes the Experiment, the fingerprint, one decision and one reason, and records one recheck per pending Score in a single locked write. It refuses if the pending list has changed since the fingerprint was taken.
• Rechecks stay in the CLI (ADR 0016). docs/guide.md's Experiments section describes the batch and the "which flip" field.

Acceptance criteria

• pending_rechecks names the flip and the Verdict it would give, for a pass and for a wrong finding.
• A batch recheck records every pending Score, each kept in its Score's history like a single recheck.
• A batch with a stale fingerprint changes nothing and says why.
• docs/guide.md describes both.