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/lib/api/precedent-library.ts

@@ -97,6 +97,15 @@ export type Halacha = {
    * UI collapses them into one review card. cluster_size === 1 → singleton. */
   cluster_id?: string;
   cluster_size?: number;
+  /* #84.2 parallel authority (present only when fetched with include_equivalents):
+   * the SAME principle stated independently in OTHER precedents — recurrence, not
+   * citation (distinct from corroboration_count). */
+  equivalents?: {
+    halacha_id: string;
+    case_number: string;
+    rule_statement: string;
+    cosine: number | null;
+  }[];
 };
 
 export type RelatedCase = {
@@ -584,7 +593,7 @@ export function useHalachotPending(
   const qs = needsFix
     ? `review_status=pending_review&exclude_low_quality=false&limit=${limit}`
     : `review_status=pending_review&exclude_low_quality=true`
-      + `&order_by_priority=true&cluster=true&limit=${limit}`;
+      + `&order_by_priority=true&cluster=true&include_equivalents=true&limit=${limit}`;
   return useQuery({
     queryKey: [...libraryKeys.halachotPending(), needsFix ? "needsfix" : "clean"],
     queryFn: async ({ signal }) => {