feat(kebab-parse-image): P6-3 caption adapter — vision LM via trait

- 신규 모듈 `crates/kebab-parse-image/src/caption.rs` 추가:
  • `caption_image(llm, bytes, lang_hint, cfg)` — `&dyn LanguageModel`
    위에서 동작. 비전 LM (예: gemma4:e4b) 이 한 문장 객관 설명
    출력. temperature=0 / seed=0 결정성.
  • `apply_caption(llm, bytes, block, lang_hint, cfg, events)` —
    `block.caption = Some(...)` 으로 채우고 ProvenanceKind::CaptionApplied
    이벤트 1건 추가. `image.caption.enabled = false` 면 클린 no-op
    (Ok(())). LM 실패 시 block.caption None 그대로 + events 미기록.
  • 다운스케일 long-edge `[128, 1536]` 클램프. PNG passthrough hot
    path 보존, 그 외는 단일 디코드 + PNG 재인코딩.
  • 한국어 / 영어 프롬프트 분기 (lang_hint=\"ko\"/\"kor\" → 한국어).
  • `ModelCaption.model_version = \"<provider>/<prompt_template_version>\"`
    (예: \"ollama/caption-v1\") — prompt 또는 모델 회귀 감사 가능.

## kebab-core / kebab-llm-local 변경

- `kebab_core::GenerateRequest` 에 `images: Vec<String>` 필드 추가.
  `#[serde(default)]` 으로 기존 wire 페이로드 / snapshot 호환.
- `kebab-llm-local::OllamaLanguageModel` 가 req.images 를 Ollama
  `images: [base64, ...]` 와이어 필드로 라우팅.
  `#[serde(skip_serializing_if = is_empty)]` 로 비어 있을 때 wire
  shape 가 pre-P6-3 와 byte-identical.

## kebab-config

- 신규 `ImageCfg.caption: CaptionCfg`:
  - `enabled: bool` (default false)
  - `max_pixels: u32` (default 768, 클램프 [128, 1536])
  - `prompt_template_version: String` (default \"caption-v1\")
- `KEBAB_IMAGE_CAPTION_{ENABLED,MAX_PIXELS,PROMPT_TEMPLATE_VERSION}`
  3종 환경변수 추가.

## Spec deviations

`tasks/HOTFIXES.md` 2026-05-02 항목 추가:
- Symptom 1: spec p6-3 시그니처가 `&dyn LanguageModel` 인데 frozen
  trait + GenerateRequest 가 vision 미지원. → trait 확장.
- Symptom 2: spec 의 cargo feature `caption` (default OFF at compile
  time) → runtime gate 1개로 통합. base64/image/kebab-llm 외 추가
  deps 없어 cargo feature 의 binary 절감 가치 미미.

p4-1 / p4-2 / p6-3 spec 의 amends 명시.

## 테스트

`cargo test -p kebab-parse-image --test caption` — 9건 + 1 ignored:
- feature gate (disabled → no-op / Err on direct call)
- happy path (block.caption Some + Provenance CaptionApplied)
- 빈 토큰 stream → empty text + caption.is_some()
- CapturingMock 으로 req.images 라우팅 검증 (base64 1개, decode 가능)
- 한국어 / 영어 프롬프트 분기 (CapturingMock 의 system 캡처)
- LM Err → block.caption None 유지 + events 미기록
- 결정성 (동일 mock 입력 → 동일 caption)
- max_pixels 클램프 (99999 → 1536, 4000×3000 PNG 다운스케일 검증)
- opt-in 통합 (실 192.168.0.47 Ollama / gemma4:e4b → \"The image is
  a solid red color.\" 검증 완료, 4.3초)

`cargo test --workspace --no-fail-fast -j 1` 전체 pass.
`cargo clippy --workspace --all-targets -- -D warnings` pass.

## 의존성 경계

- 추가 deps: `kebab-llm` (trait 만), `base64` (이미 P6-2 에서 추가).
- dev-deps: `kebab-llm/mock` 으로 `MockLanguageModel`,
  `kebab-llm-local` (통합 테스트 전용 — 런타임 deps 에는 없음).
- forbidden 침범 없음: `kebab-source-fs / parse-md / normalize /
  chunk / store-* / embed* / search / rag / UI` 미참조.

contract: docs/superpowers/specs/2026-04-27-kebab-final-form-design.md
sections: §3.4 ImageRefBlock.caption, §3.7a ModelCaption, §9.1
caption (model-generated, low trust).

tasks/HOTFIXES.md

@@ -14,6 +14,30 @@ historical contract that was implemented; this file accumulates the
 deltas so phase 5+ readers can find the live behavior without diffing
 git history.
 
+## 2026-05-02 — P6-3 caption: GenerateRequest.images + cargo feature dropped
+
+**Discovered**: P6-3 implementation start.
+
+**Symptom 1**: `tasks/p6/p6-3-caption-adapter.md` § Public surface declares `caption_image(llm: &dyn kebab_core::LanguageModel, ...)`, but the frozen `LanguageModel` trait + `GenerateRequest` from p4-1 carry no vision input. The spec's behavior contract ("the adapter is responsible for rendering the prompt to wire") implicitly relied on a trait extension that p4-1 never specced.
+
+**Symptom 2**: Spec § Definition of Done asks for `cargo check -p kebab-parse-image --features caption` — i.e. a cargo feature gate. The captioning module's only extra deps are `base64` + `image` + the `kebab-llm` trait, all already pulled in by P6-2. A cargo feature would only complicate the build matrix without saving meaningful binary weight.
+
+**Root cause**: Two small spec gaps that resolve cleanly together — extend the `LanguageModel` trait once for vision routing, and collapse compile-time + runtime gating into a single runtime gate.
+
+**Fix** (PR #34, feat/p6-3-caption-adapter):
+- `kebab-core::GenerateRequest` gains an `images: Vec<String>` field (`#[serde(default)]` for backward compat with pre-P6 wire payloads / snapshots). Empty for the text-only RAG path; populated with one or more base64 strings by vision-aware callers.
+- `kebab-llm-local::OllamaLanguageModel` routes `req.images` onto the wire as `images: [base64, ...]` (Ollama's vision channel). The wire shape stays byte-identical for empty `images` because the field uses `#[serde(skip_serializing_if = "<[String]>::is_empty")]`.
+- `kebab-parse-image::caption` module: `caption_image` / `apply_caption` build `GenerateRequest { images: vec![b64], temperature: 0.0, seed: 0, ... }` and accept any `&dyn LanguageModel`. Korean / English prompt branch picked from `lang_hint`.
+- Cargo feature `caption` is **not** introduced — the runtime gate `config.image.caption.enabled = false` (default OFF) suffices.
+- All existing `GenerateRequest { ... }` literals (kebab-rag, kebab-llm tests, kebab-llm-local tests) gained `images: Vec::new()` to satisfy the new field.
+
+**Trust note**: Captions stay explicitly model-generated. `ModelCaption.model_version` carries `"<provider>/<prompt_template_version>"` (e.g. `"ollama/caption-v1"`) so a regression in either prompt or model is auditable from the wire.
+
+**Amends**:
+- tasks/p4/p4-1-llm-trait.md (`GenerateRequest` schema gained `images: Vec<String>`).
+- tasks/p4/p4-2-ollama-adapter.md (request body now optionally includes `images: [...]`).
+- tasks/p6/p6-3-caption-adapter.md ("Definition of Done" cargo feature `caption` dropped; runtime gate is the only feature gate).
+
 ## 2026-05-02 — P6-2 default OCR engine: Tesseract → Ollama-vision
 
 **Discovered**: P6-2 implementation start.