review(p6-1): 회차 3 cosmetic 정리

- src/dims.rs: `with_guessed_format()` 의 `map_err(...)` 를 `.context()?`
  로 정리. 회차 2 의 `match Some/None` → `.context()?` 정리와 호출
  스타일 통일.
- src/lib.rs: `(*format).to_string()` → `format.to_string()`. `format` 이
  `&&'static str` 이라 명시 deref 없이 자동 호출 가능.
- tests/common/mod.rs: `ImageFixture::workspace_root` / `config` 가시성을
  `pub` → 모듈-비공개로 축소. 외부 호출자가 두 필드를 직접 읽지 않고
  `ctx()` 만 사용함.

cargo test -p kebab-parse-image — 16건 pass.
cargo clippy -p kebab-parse-image --all-targets -- -D warnings — pass.

crates/kebab-parse-image/src/dims.rs

@@ -35,7 +35,7 @@ pub(crate) enum DimOutcome {
 pub(crate) fn probe(bytes: &[u8]) -> Result<DimOutcome> {
     let reader = ImageReader::new(Cursor::new(bytes))
         .with_guessed_format()
-        .map_err(|e| anyhow::anyhow!("io error guessing format: {e}"))?;
+        .context("reading image header")?;
 
     let format = reader
         .format()

crates/kebab-parse-image/src/lib.rs

@@ -85,7 +85,7 @@ impl Extractor for ImageExtractor {
                 let mut dims = Map::new();
                 dims.insert("w".into(), Value::Number((*width).into()));
                 dims.insert("h".into(), Value::Number((*height).into()));
-                dims.insert("format".into(), Value::String((*format).to_string()));
+                dims.insert("format".into(), Value::String(format.to_string()));
                 (
                     SourceSpan::Region {
                         x: 0,

crates/kebab-parse-image/tests/common/mod.rs

@@ -210,8 +210,8 @@ pub fn corrupt_png() -> Vec<u8> {
 /// per §4.2 — this matches what `kebab-source-fs` does in production.
 pub struct ImageFixture {
     pub asset: RawAsset,
-    pub workspace_root: PathBuf,
-    pub config: ExtractConfig,
+    workspace_root: PathBuf,
+    config: ExtractConfig,
 }
 
 impl ImageFixture {