feat(eval): fb-41 multi-hop golden set + spec/plan

PR-1 of fb-41 multi-hop RAG (spec: docs/superpowers/specs/2026-05-25-
p9-fb-41-multi-hop-rag-design.md, plan: docs/superpowers/plans/2026-
05-25-p9-fb-41-multi-hop-rag.md).

XL 작업의 첫 PR — baseline 측정 anchor 만 추가. RAG pipeline 미변경,
fixture file + parse 회귀 핀.

사용자 결정 4 axis (2026-05-25):
- approach: query decomposition (LLM 서브-질문)
- trigger: explicit `--multi-hop` flag
- MVP scope: dynamic N-hop (LLM 이 depth 결정, decompose seed +
  ReAct-style decide loop hybrid)
- eval: multi-hop golden set 먼저 (본 PR)

본 PR:
- `fixtures/multi_hop_golden.yaml` 신규. 15 question (5 cross-doc +
  5 intra-doc + 5 single-fact negative). 기존 `GoldenQuery` struct
  그대로 사용 — 별 loader / type 변경 없음. `expected_chunk_ids`
  비어 있어 curator 가 `kebab ingest` 후 채울 수 있는 template
  형태. `must_contain` 으로 baseline 측정 가능 (P5-2 metric).
- `crates/kebab-eval/tests/loader.rs::loads_multi_hop_golden_fixture`
  신규 회귀 핀. fixture parse OK + 15 question + 5/5/5 bucket
  분포 + 모든 question 에 must_contain 최소 1 개.

baseline 측정 protocol (별 run, commit 에 artifact 안 포함):
1. v0.17.2 binary 로 single-pass `kebab eval run --fixture
   multi_hop_golden.yaml` 실행
2. P@5, P@10, must_contain pass rate, citation_coverage 캡처
3. PR-3 (dynamic iter 머지) 후 동일 fixture + `multi_hop=true` 로
   재실행 → Δ 비교

PR 분할 6 단계 (plan 참조): PR-1 (본 PR — fixture only), PR-2
(RagPipeline::ask_multi_hop fixed depth=2), PR-3 (dynamic iter),
PR-4 (CLI flag + wire), PR-5 (MCP + SKILL.md), PR-6 (TUI toggle +
trace render). 마지막 PR 후 v0.18.0 cut.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-eval/tests/loader.rs

@@ -38,7 +38,44 @@ fn loads_minimal_well_formed_yaml() {
     assert_eq!(qs[1].difficulty.as_deref(), Some("easy"));
 }
 
-// ── 2. duplicate IDs error lists every offender (sorted, deduplicated) ───────
+// ── 2. fb-41 multi-hop golden fixture loads + sanity-checks ─────────────────
+
+/// fb-41 baseline + post-merge Δ measurement fixture. The shared
+/// loader must accept `fixtures/multi_hop_golden.yaml` and the bucket
+/// distribution must stay 5 cross-doc + 5 intra-doc + 5 single-fact
+/// negative — curators dropping or re-id'ing a question hit a clear
+/// failure mode here before it reaches the runner.
+#[test]
+fn loads_multi_hop_golden_fixture() {
+    let path = std::path::Path::new(env!("CARGO_MANIFEST_DIR"))
+        .join("..")
+        .join("..")
+        .join("fixtures")
+        .join("multi_hop_golden.yaml");
+    let qs = load_golden_set(&path).expect("multi_hop_golden.yaml must parse");
+
+    assert_eq!(qs.len(), 15, "fb-41 fixture must have 15 questions");
+
+    let cross_doc = qs.iter().filter(|q| q.id.starts_with("mh-c-")).count();
+    let intra_doc = qs.iter().filter(|q| q.id.starts_with("mh-i-")).count();
+    let single = qs.iter().filter(|q| q.id.starts_with("mh-s-")).count();
+    assert_eq!(cross_doc, 5, "expected 5 mh-c-* (cross-doc) questions");
+    assert_eq!(intra_doc, 5, "expected 5 mh-i-* (intra-doc) questions");
+    assert_eq!(single, 5, "expected 5 mh-s-* (single-fact negative) questions");
+
+    // Every question carries at least one `must_contain` so the
+    // rule-based answer-correctness metric (P5-2) has a signal even
+    // before `expected_chunk_ids` are filled in.
+    for q in &qs {
+        assert!(
+            !q.must_contain.is_empty(),
+            "{}: must_contain is empty — baseline measurement needs a signal",
+            q.id
+        );
+    }
+}
+
+// ── 3. duplicate IDs error lists every offender (sorted, deduplicated) ───────
 
 #[test]
 fn rejects_duplicate_ids() {