feat(rag): fb-41 PR-3a — HopRecord wire + RagCfg multi-hop knobs

PR-3 의 분할 첫 PR. wire additive (HopRecord + HopKind + Answer.hops
field) + RagCfg 의 multi_hop_* 3 노브. RAG pipeline 동작 미변경 —
모든 Answer literal 의 `hops = None`. PR-3b (후속) 가 ask_multi_hop
의 happy path 에서 dynamic decide loop 구현 + hops trace 채움.

분할 이유: 원래 PR-3 가 wire + cfg + decide loop + ScriptedLm +
helper refactor + 5+ tests 단일 PR 였는데 ~1500 줄 단일 patch 가
review 부담 + 회기 위험 ↑. additive foundation 부터 ship 후 decide
loop 별 PR — 사용자 결정 (2026-05-25).

- `kebab_core::HopRecord` (iter, kind, sub_queries,
  context_chunks_added, forced_stop, llm_call_ms) + `HopKind`
  (Decompose / Decide / Synthesize) — wire-additive shape.
- `kebab_core::Answer.hops: Option<Vec<HopRecord>>` —
  `#[serde(default, skip_serializing_if = "Option::is_none")]`,
  single-pass / refusal path 는 None, PR-3b 의 multi-hop happy
  path 가 Some.
- `kebab_config::RagCfg` 에 3 신규 노브:
  - `multi_hop_max_depth: u32` (default 3)
  - `multi_hop_max_sub_queries_per_iter: u32` (default 5)
  - `multi_hop_max_pool_chunks: u32` (default 30)
  3 모두 `#[serde(default)]` + env override
  (`KEBAB_RAG_MULTI_HOP_MAX_*`) + legacy parse 핀
  (`LEGACY_PRE_TIMEOUT_TOML` 공유).
- 9 Answer literal site (pipeline.rs ×6 + kebab-cli + kebab-tui
  tests + kebab-eval test) 에 `hops: None` 명시 추가. exhaustive
  field check 가 자동 guard — 빠진 site 시 compile fail.
- plan 의 PR-3 단락 → PR-3a / PR-3b 분할 명시 + scope 정정.

Tests (163 passing across kebab-config + kebab-core + kebab-rag):
- 5 신규 multi-hop knob test (default / env override / legacy parse).
- 기존 50+57+31+19+3+3 test 모두 hops:None 추가 후도 통과.

Wire 영향: `answer.v1` 의 optional `hops` 필드 — `skip_serializing_
if = None` 이라 single-pass response 에 emit 안 됨. wire breaking
아님, JSON Schema 갱신은 PR-3b 또는 PR-4 (실제 emit 시점).

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

crates/kebab-rag/src/pipeline.rs

@@ -531,6 +531,11 @@ impl RagPipeline {
             created_at: OffsetDateTime::now_utc(),
             conversation_id: opts.conversation_id.clone(),
             turn_index: opts.turn_index,
+            // p9-fb-41 Step 2 of PR-3: every Answer literal carries
+            // `hops`. Single-pass + refusal paths leave it `None`;
+            // only the multi-hop happy path will set `Some(...)` in
+            // Step 5 once the decide loop populates a hop trace.
+            hops: None,
         };
 
         // Drop the moved `finish_reason` early into a tracing breadcrumb; the
@@ -843,6 +848,11 @@ impl RagPipeline {
             created_at: OffsetDateTime::now_utc(),
             conversation_id: opts.conversation_id.clone(),
             turn_index: opts.turn_index,
+            // p9-fb-41 Step 2 of PR-3: every Answer literal carries
+            // `hops`. Single-pass + refusal paths leave it `None`;
+            // only the multi-hop happy path will set `Some(...)` in
+            // Step 5 once the decide loop populates a hop trace.
+            hops: None,
         };
 
         tracing::debug!(
@@ -979,6 +989,11 @@ impl RagPipeline {
             created_at: OffsetDateTime::now_utc(),
             conversation_id: opts.conversation_id.clone(),
             turn_index: opts.turn_index,
+            // p9-fb-41 Step 2 of PR-3: every Answer literal carries
+            // `hops`. Single-pass + refusal paths leave it `None`;
+            // only the multi-hop happy path will set `Some(...)` in
+            // Step 5 once the decide loop populates a hop trace.
+            hops: None,
         };
         if let Some(sink) = &opts.stream_sink {
             let _ = sink.send(StreamEvent::Final {
@@ -1100,6 +1115,11 @@ impl RagPipeline {
             created_at: OffsetDateTime::now_utc(),
             conversation_id: opts.conversation_id.clone(),
             turn_index: opts.turn_index,
+            // p9-fb-41 Step 2 of PR-3: every Answer literal carries
+            // `hops`. Single-pass + refusal paths leave it `None`;
+            // only the multi-hop happy path will set `Some(...)` in
+            // Step 5 once the decide loop populates a hop trace.
+            hops: None,
         };
         if let Err(e) = self.docs.put_answer(&answer, query, None) {
             tracing::warn!(target: "kebab-rag", error = %e, "kb-rag: put_answer (NoChunks) failed");
@@ -1182,6 +1202,11 @@ impl RagPipeline {
             created_at: OffsetDateTime::now_utc(),
             conversation_id: opts.conversation_id.clone(),
             turn_index: opts.turn_index,
+            // p9-fb-41 Step 2 of PR-3: every Answer literal carries
+            // `hops`. Single-pass + refusal paths leave it `None`;
+            // only the multi-hop happy path will set `Some(...)` in
+            // Step 5 once the decide loop populates a hop trace.
+            hops: None,
         };
         if let Err(e) = self.docs.put_answer(&answer, query, None) {
             tracing::warn!(target: "kebab-rag", error = %e, "kb-rag: put_answer (ScoreGate) failed");
@@ -1789,6 +1814,7 @@ mod stream_event_serde_tests {
             created_at: datetime!(2026-05-09 12:00:00 UTC),
             conversation_id: None,
             turn_index: None,
+            hops: None,
         };
         let ev = StreamEvent::Final { answer };
         let v = serde_json::to_value(&ev).unwrap();