feat(app): App::fetch chunk mode + markdown serializer (fb-35)

Chunk mode + +-N context. doc / span modes return placeholder
errors (filled by subsequent tasks). fmt_canonical_to_markdown
helper introduced now since doc mode (Task 4) consumes it.
Errors are typed StructuredError so classify preserves
chunk_not_found / doc_not_found through the wire layer.

Adds SqliteStore::list_chunk_ids_for_doc so the facade can derive
+-N neighbors without leaking direct rusqlite usage into kebab-app.

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

crates/kebab-app/tests/fetch_integration.rs

@@ -0,0 +1,85 @@
+//! p9-fb-35 App::fetch integration tests.
+
+mod common;
+
+use kebab_app::App;
+use kebab_core::{FetchKind, FetchOpts, FetchQuery};
+
+fn open(env: &common::TestEnv) -> App {
+    env.app()
+}
+
+#[test]
+fn fetch_chunk_returns_target_only_when_no_context() {
+    let env = common::TestEnv::new();
+    common::ingest_md(&env, "a.md", "# Title\n\nFirst paragraph.\n\n## Section\n\nSecond.\n");
+    let app = open(&env);
+
+    // Find a chunk via search to obtain its id.
+    let q = kebab_core::SearchQuery {
+        text: "First".to_string(),
+        mode: kebab_core::SearchMode::Lexical,
+        k: 1,
+        filters: kebab_core::SearchFilters::default(),
+    };
+    let hits = app.search(q).unwrap();
+    let chunk_id = hits[0].chunk_id.clone();
+
+    let result = app
+        .fetch(FetchQuery::Chunk(chunk_id), FetchOpts::default())
+        .unwrap();
+    assert_eq!(result.kind, FetchKind::Chunk);
+    assert!(result.chunk.is_some(), "target chunk populated");
+    assert!(result.context_before.is_empty());
+    assert!(result.context_after.is_empty());
+    assert!(!result.truncated);
+}
+
+#[test]
+fn fetch_chunk_with_context_returns_neighbors() {
+    let env = common::TestEnv::new();
+    let body = "# H1\n\nA1\n\n# H2\n\nA2\n\n# H3\n\nA3\n\n# H4\n\nA4\n\n# H5\n\nA5\n";
+    common::ingest_md(&env, "multi.md", body);
+    let app = env.app();
+
+    let q = kebab_core::SearchQuery {
+        text: "A3".to_string(),
+        mode: kebab_core::SearchMode::Lexical,
+        k: 1,
+        filters: kebab_core::SearchFilters::default(),
+    };
+    let hits = app.search(q).unwrap();
+    let chunk_id = hits[0].chunk_id.clone();
+
+    let result = app
+        .fetch(
+            FetchQuery::Chunk(chunk_id),
+            FetchOpts {
+                context: Some(2),
+                max_tokens: None,
+            },
+        )
+        .unwrap();
+    assert_eq!(result.kind, FetchKind::Chunk);
+    assert!(result.chunk.is_some());
+    let total = result.context_before.len() + result.context_after.len();
+    assert!(total >= 1, "at least one neighbor expected");
+    assert!(total <= 4, "context capped at +-2 ⇒ max 4 neighbors");
+}
+
+#[test]
+fn fetch_chunk_unknown_id_returns_chunk_not_found() {
+    let env = common::TestEnv::new();
+    let app = env.app();
+    let err = app
+        .fetch(
+            FetchQuery::Chunk(kebab_core::ChunkId("nonexistent-id".to_string())),
+            FetchOpts::default(),
+        )
+        .unwrap_err();
+    let msg = err.to_string();
+    assert!(
+        msg.contains("chunk_not_found") || msg.contains("nonexistent-id"),
+        "expected chunk_not_found error, got: {msg}"
+    );
+}