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/authority_rerank_test.go

@@ -0,0 +1,96 @@
+package ucca
+
+import "testing"
+
+func bindingRes(label, reg, jur string, score float64) LegalSearchResult {
+	return LegalSearchResult{ArticleLabel: label, RegulationShort: reg, SourceClass: "binding_law", AuthorityWeight: 100, Jurisdiction: jur, Score: score}
+}
+
+func guidanceRes(label, reg string, score float64) LegalSearchResult {
+	return LegalSearchResult{ArticleLabel: label, RegulationShort: reg, SourceClass: "supervisory_guidance", AuthorityWeight: 70, Jurisdiction: "EU", Score: score}
+}
+
+func foreignRes(label string, score float64) LegalSearchResult {
+	return LegalSearchResult{ArticleLabel: label, RegulationShort: "RevDSG", SourceClass: "foreign_law", AuthorityWeight: 0, Jurisdiction: "CH", Score: score}
+}
+
+// Acceptance criteria (Phase 1) expressed as ordering tests.
+func TestRerankByAuthority_Acceptance(t *testing.T) {
+	t.Run("guidance does not overtake semantically competitive binding", func(t *testing.T) {
+		out := rerankByAuthority("Was gilt hier?", []LegalSearchResult{
+			guidanceRes("ENISA Mapping", "ENISA", 0.72),
+			bindingRes("CRA Anhang I", "CRA", "EU", 0.66),
+		})
+		if out[0].RegulationShort != "CRA" {
+			t.Fatalf("binding must rank first over competitive guidance, got %q", out[0].RegulationShort)
+		}
+	})
+
+	t.Run("foreign law demoted on DE/EU question but kept", func(t *testing.T) {
+		in := []LegalSearchResult{foreignRes("RevDSG Art 1", 0.85), bindingRes("Art. 9 DSGVO", "DSGVO", "EU", 0.62)}
+		out := rerankByAuthority("Welche Daten sind besonders geschuetzt?", in)
+		if out[0].RegulationShort != "DSGVO" {
+			t.Fatalf("binding EU must beat foreign on a DE/EU query, got %q", out[0].RegulationShort)
+		}
+		if len(out) != 2 {
+			t.Fatalf("foreign law must be kept, got len=%d", len(out))
+		}
+	})
+
+	t.Run("off-domain binding demoted but not removed", func(t *testing.T) {
+		in := []LegalSearchResult{
+			bindingRes("Art. 13 EU MDR", "MDR", "EU", 0.70),
+			bindingRes("Art. 13 CRA", "CRA", "EU", 0.60),
+		}
+		out := rerankByAuthority("Welche Pflichten hat der Hersteller von Produkten mit digitalen Elementen?", in)
+		if out[0].RegulationShort != "CRA" {
+			t.Fatalf("on-domain CRA must beat off-domain MDR, got %q", out[0].RegulationShort)
+		}
+		if len(out) != 2 {
+			t.Fatalf("off-domain MDR must be kept, got len=%d", len(out))
+		}
+	})
+
+	t.Run("same-regime binding wins over guidance", func(t *testing.T) {
+		out := rerankByAuthority("Was gilt hier?", []LegalSearchResult{
+			bindingRes("Art. 13 CRA", "CRA", "EU", 0.70),
+			guidanceRes("ENISA Mapping", "ENISA", 0.60),
+		})
+		if out[0].RegulationShort != "CRA" {
+			t.Fatalf("binding must win, got %q", out[0].RegulationShort)
+		}
+	})
+
+	t.Run("BDSG Teil 3 demoted below DSGVO on general DP question", func(t *testing.T) {
+		in := []LegalSearchResult{
+			bindingRes("§ 48 BDSG", "BDSG", "DE", 0.70), // Teil 3 (law enforcement)
+			bindingRes("Art. 9 DSGVO", "DSGVO", "EU", 0.62),
+		}
+		out := rerankByAuthority("Was sind besondere Kategorien personenbezogener Daten?", in)
+		if out[0].RegulationShort != "DSGVO" {
+			t.Fatalf("DSGVO must beat BDSG Teil 3 on a general DP question, got %q", out[0].RegulationShort)
+		}
+	})
+
+	t.Run("nothing is dropped and topic amplifies", func(t *testing.T) {
+		in := []LegalSearchResult{
+			guidanceRes("ENISA", "ENISA", 0.72),
+			bindingRes("CRA Anhang I", "CRA", "EU", 0.66),
+			foreignRes("RevDSG", 0.5),
+		}
+		out := rerankByAuthority("Anforderungen an Security Updates?", in)
+		if len(out) != len(in) {
+			t.Fatalf("rerank must preserve all results, got %d want %d", len(out), len(in))
+		}
+		if out[0].ArticleLabel != "CRA Anhang I" {
+			t.Fatalf("topic+authority must lift CRA Anhang I to top, got %q", out[0].ArticleLabel)
+		}
+	})
+
+	t.Run("single result returned unchanged", func(t *testing.T) {
+		in := []LegalSearchResult{bindingRes("Art. 1 CRA", "CRA", "EU", 0.5)}
+		if out := rerankByAuthority("x", in); len(out) != 1 {
+			t.Fatalf("len=%d", len(out))
+		}
+	})
+}