feat(halacha): equivalent-halacha (parallel-authority) links across precedents

Cross-precedent recurrence of a principle is real but is NOT citation
corroboration (X11) — the 5 candidate pairs have ZERO citations between their
precedents. Recording them in halacha_citation_corroboration would fabricate
citation data and inflate corroboration_count. This adds a proper, separate
halacha-level link for parallel authority.

Schema (V28): equivalent_halachot — symmetric (halacha_a < halacha_b, CHECK +
UNIQUE), non-citation, cross-precedent-only. ON DELETE CASCADE.

db.py:
- link_equivalent_halachot (idempotent; rejects same-id and SAME-precedent pairs
  — parallel authority is cross-precedent by definition), unlink, and
  list_equivalent_for_halacha.
- list_halachot gains include_equivalents → _annotate_equivalents attaches an
  `equivalents` list (both directions) per row.

API: include_equivalents on GET /api/halachot; GET/POST/DELETE
/api/halachot/{id}/equivalents for the chair to view/link/unlink manually.

scripts/halacha_batch_reconcile.py: --link records found cross-precedent pairs
as equivalent_halachot (non-destructive, idempotent).

web-ui: Halacha.equivalents type; the clean review queue fetches
include_equivalents; the review card shows a gold "עיקרון מקביל ב-N" badge + an
expandable list (case + rule + similarity) labeled "אסמכתה מקבילה — לא ציטוט".

Populated the 5 reviewed pairs (chair decision: keep all + link as parallel
authority). Verified: 5 rows; the 1023-20 hub annotates 3 of its halachot with
equivalents; tsc --noEmit exits 0.

Invariants: G1 (model recurrence at source in its own table, not by abusing the
citator); G2 (no parallel path — extends list_halachot); citator integrity
preserved (corroboration stays citation-only).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web-ui/src/components/precedents/halacha-review-panel.tsx

@@ -68,7 +68,9 @@ function HalachaCard({
   onSave: (patch: Partial<EditState>) => Promise<void>;
 }) {
   const variants = h.variants ?? [];
+  const equivalents = h.equivalents ?? [];
   const [showVariants, setShowVariants] = useState(false);
+  const [showEquiv, setShowEquiv] = useState(false);
   const [editing, setEditing] = useState(false);
   const [draft, setDraft] = useState<EditState>({
     rule_statement: h.rule_statement,
@@ -122,6 +124,12 @@ function HalachaCard({
               +{variants.length} וריאנטים
             </Badge>
           )}
+          {equivalents.length > 0 && (
+            <Badge variant="outline"
+              className="text-[0.65rem] bg-gold-wash text-gold-deep border-gold/40">
+              עיקרון מקביל ב-{equivalents.length}
+            </Badge>
+          )}
           <CorroborationBadge halacha={h} />
         </span>
       </div>
@@ -220,6 +228,38 @@ function HalachaCard({
         </div>
       )}
 
+      {equivalents.length > 0 && (
+        <div className="rounded-md border border-gold/30 bg-gold-wash/40">
+          <button
+            type="button"
+            onClick={() => setShowEquiv((v) => !v)}
+            className="w-full flex items-center gap-2 px-3 py-2 text-[0.72rem] text-gold-deep hover:bg-gold-wash/70 transition-colors"
+            aria-expanded={showEquiv}
+          >
+            {showEquiv ? <ChevronDown className="w-3.5 h-3.5" /> : <ChevronLeft className="w-3.5 h-3.5" />}
+            <span className="font-medium">
+              עיקרון מקביל ב-{equivalents.length} החלטות אחרות (אסמכתה מקבילה)
+            </span>
+            <span className="me-auto text-ink-muted">לא ציטוט — הישנות עצמאית</span>
+          </button>
+          {showEquiv && (
+            <ul className="px-4 pb-3 pt-1 space-y-2">
+              {equivalents.map((e) => (
+                <li key={e.halacha_id} className="text-[0.78rem] text-ink-soft leading-relaxed border-r-2 border-gold/30 pr-3" dir="rtl">
+                  <span className="font-semibold text-navy">{cleanCitation(e.case_number)}</span>
+                  {" — "}{e.rule_statement}
+                  {e.cosine != null && (
+                    <span className="text-[0.65rem] text-ink-muted tabular-nums ms-2">
+                      (דמיון {e.cosine.toFixed(2)})
+                    </span>
+                  )}
+                </li>
+              ))}
+            </ul>
+          )}
+        </div>
+      )}
+
       <div className="flex items-center gap-2 justify-end pt-1 border-t border-rule-soft">
         {editing ? (
           <>