feat(app): App::fetch span mode + PDF/audio rejection (fb-35)

Line-based slice over fmt_canonical_to_markdown output.
PDF / audio source_type → span_not_supported StructuredError.
Out-of-range line_end clamps to total; effective_end reflects
post-budget trim. invalid_input on zero / inverted bounds.

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

crates/kebab-app/tests/fetch_integration.rs

@@ -153,3 +153,100 @@ fn fetch_doc_with_max_tokens_truncates() {
     let text = result.text.expect("doc text");
     assert!(text.chars().count() <= 100, "trimmed text len {}", text.chars().count());
 }
+
+#[test]
+fn fetch_span_returns_line_range() {
+    let env = common::TestEnv::new();
+    // Use a list so the canonical-to-markdown roundtrip emits 5
+    // single-line entries joined by `\n` (paragraphs would be joined by
+    // `\n\n`, and CommonMark soft breaks inside one paragraph collapse to
+    // spaces — see crates/kebab-parse-md/src/blocks.rs `Event::SoftBreak`).
+    let body = "- Line one.\n- Line two.\n- Line three.\n- Line four.\n- Line five.\n";
+    common::ingest_md(&env, "lines.md", body);
+    let app = env.app();
+
+    let q = kebab_core::SearchQuery {
+        text: "Line".to_string(),
+        mode: kebab_core::SearchMode::Lexical,
+        k: 1,
+        filters: kebab_core::SearchFilters::default(),
+    };
+    let hits = app.search(q).unwrap();
+    let doc_id = hits[0].doc_id.clone();
+
+    let result = app
+        .fetch(
+            FetchQuery::Span {
+                doc_id,
+                line_start: 2,
+                line_end: 4,
+            },
+            FetchOpts::default(),
+        )
+        .unwrap();
+    assert_eq!(result.kind, FetchKind::Span);
+    let text = result.text.expect("span text");
+    let line_count = text.lines().count();
+    assert_eq!(line_count, 3, "span should be 3 lines: {text:?}");
+    assert_eq!(result.line_start, Some(2));
+    assert_eq!(result.line_end, Some(4));
+    assert_eq!(result.effective_end, Some(4));
+    assert!(!result.truncated);
+}
+
+#[test]
+fn fetch_span_clamps_line_end_when_out_of_range() {
+    let env = common::TestEnv::new();
+    common::ingest_md(&env, "short.md", "Line one.\nLine two.\n");
+    let app = env.app();
+    let q = kebab_core::SearchQuery {
+        text: "Line".to_string(),
+        mode: kebab_core::SearchMode::Lexical,
+        k: 1,
+        filters: kebab_core::SearchFilters::default(),
+    };
+    let hits = app.search(q).unwrap();
+    let doc_id = hits[0].doc_id.clone();
+
+    let result = app
+        .fetch(
+            FetchQuery::Span {
+                doc_id,
+                line_start: 1,
+                line_end: 999,
+            },
+            FetchOpts::default(),
+        )
+        .unwrap();
+    let text = result.text.expect("span text");
+    let actual_lines = text.lines().count();
+    assert_eq!(result.effective_end, Some(actual_lines as u32));
+    assert!(actual_lines < 999);
+}
+
+#[test]
+fn fetch_span_invalid_input_when_zero_lines() {
+    let env = common::TestEnv::new();
+    common::ingest_md(&env, "a.md", "Line one.\n");
+    let app = env.app();
+    let q = kebab_core::SearchQuery {
+        text: "Line".to_string(),
+        mode: kebab_core::SearchMode::Lexical,
+        k: 1,
+        filters: kebab_core::SearchFilters::default(),
+    };
+    let hits = app.search(q).unwrap();
+    let doc_id = hits[0].doc_id.clone();
+
+    let err = app
+        .fetch(
+            FetchQuery::Span {
+                doc_id,
+                line_start: 0,
+                line_end: 0,
+            },
+            FetchOpts::default(),
+        )
+        .unwrap_err();
+    assert!(err.to_string().contains("invalid_input"), "got: {err}");
+}