feat(ai-sdk): authority-aware re-ranking for legal RAG retrieval (Phase 1)

Re-orders /sdk/v1/rag/search results so binding law from the matching
jurisdiction and domain ranks above guidance, foreign and off-domain law —
without dropping anything (guidance stays as interpretation context).
Internal-only: response schema is unchanged (json:"-" fields), so every
consumer benefits without a contract change.

- authority.go: classifyAuthority / queryDomain / chunkDomain / scopeClass /
  topic ontology. Tagged payload (authority_weight/source_class/jurisdiction)
  wins; deterministic fallback via category + name markers for the untagged corpus.
- authority_rerank.go: rerankByAuthority. final = semantic + authority +
  jurisdiction + domain + scope + topic; the authority score is written back to
  Score so the multi-collection advisor merge preserves the order.
- legal_rag_client: stratified retrieval — the binding-law pool AUGMENTS the
  semantic pool (mergeDedupHits), then re-rank.
- legal_rag_http: searchBinding (source_class filter) + shared doPointsSearch.
- table-driven tests for authority/domain/scope/topic + rerank acceptance +
  a stratified-binding integration test. go test -race green.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

ai-compliance-sdk/internal/ucca/legal_rag_http.go

@@ -185,6 +185,27 @@ func (c *LegalRAGClient) searchDense(ctx context.Context, collection string, emb
 		searchReq.Filter = &qdrantFilter{Should: conditions}
 	}
 
+	return c.doPointsSearch(ctx, collection, searchReq)
+}
+
+// searchBinding fetches the top binding_law hits (authority-stratified pool) so the
+// obligation source is always a candidate even when guidance dominates semantically.
+// It AUGMENTS the semantic pool — guidance is preserved as interpretation context.
+func (c *LegalRAGClient) searchBinding(ctx context.Context, collection string, embedding []float64, topK int) ([]qdrantSearchHit, error) {
+	searchReq := qdrantSearchRequest{
+		Vector:      embedding,
+		Limit:       topK,
+		WithPayload: true,
+		Filter: &qdrantFilter{Must: []qdrantCondition{
+			{Key: "source_class", Match: qdrantMatch{Value: "binding_law"}},
+		}},
+	}
+
+	return c.doPointsSearch(ctx, collection, searchReq)
+}
+
+// doPointsSearch issues a POST /points/search and decodes the hits.
+func (c *LegalRAGClient) doPointsSearch(ctx context.Context, collection string, searchReq qdrantSearchRequest) ([]qdrantSearchHit, error) {
 	jsonBody, err := json.Marshal(searchReq)
 	if err != nil {
 		return nil, fmt.Errorf("failed to marshal search request: %w", err)