ux-pain-digest retune + per-field granularity after feedback-category unification

AFTER VENUS-188 lands + real post-flip volume accrues (do NOT tune on today's near-zero data). Two digest refinements coder flagged when the friction mechanisms fold into action='error-shown': (1) THRESHOLD RETUNE — ux-pain-digest.ts MIN_COUNT=10/CRITICAL_COUNT=100 were calibrated on guardrail-toast-only volume; folding in action-error + native-validation + client-validation raises counts and native-validation (every blocked-submit field) could dominate top-15 and bury higher-value action-error/guardrail patterns. Retune thresholds and/or weight/segment by detail.mechanism so low-value native-validation noise doesn't drown real errors. (2) PER-FIELD GRANULARITY — native-validation Spanish messages are generic per constraint ('Campo obligatorio sin completar.' for all required fails), so the digest's GROUP BY message collapses same-constraint fails across different fields into one row; the field lives in detail.field. To get per-field digest rows the digest query must read detail.field/detail.mechanism. Digest-side change, db-venus/PM scope. Gate: needs a few weeks of real post-flip data first.