feat(app): App::search_with_opts + SearchResponse (fb-34)

Budget loop: snippet shorten → k pop → ≥1 hit floor. Cursor
encode/decode threads corpus_revision; mismatch surfaces as
stale_cursor anyhow error. App::search retained as thin wrapper.

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

crates/kebab-app/tests/search_budget_integration.rs

@@ -0,0 +1,121 @@
+//! p9-fb-34: App::search_with_opts integration tests.
+
+mod common;
+
+use kebab_app::SearchResponse;
+use kebab_core::{SearchFilters, SearchMode, SearchOpts, SearchQuery};
+
+fn lex(text: &str, k: usize) -> SearchQuery {
+    SearchQuery {
+        text: text.to_string(),
+        mode: SearchMode::Lexical,
+        k,
+        filters: SearchFilters::default(),
+    }
+}
+
+#[test]
+fn search_with_opts_no_budget_matches_search() {
+    let env = common::TestEnv::new();
+    common::ingest_md(&env, "a.md", "# T\n\napples are red\n");
+    let app = env.app();
+
+    let baseline = app.search(lex("apples", 5)).unwrap();
+    let resp: SearchResponse = app
+        .search_with_opts(lex("apples", 5), SearchOpts::default())
+        .unwrap();
+
+    assert_eq!(resp.hits.len(), baseline.len());
+    assert!(!resp.truncated);
+    assert!(resp.next_cursor.is_none(), "k=5 against 1 doc → no next page");
+}
+
+#[test]
+fn budget_truncates_snippets_when_below_threshold() {
+    let env = common::TestEnv::new();
+    let body: String = "rust ownership is a memory model. ".repeat(10);
+    common::ingest_md(&env, "a.md", &format!("# T\n\n{body}\n"));
+    let app = env.app();
+
+    let unrestricted = app.search(lex("rust", 5)).unwrap();
+    let unrestricted_chars: usize = unrestricted.iter().map(|h| h.snippet.chars().count()).sum();
+
+    let resp = app
+        .search_with_opts(
+            lex("rust", 5),
+            SearchOpts {
+                max_tokens: Some(50),
+                snippet_chars: None,
+                cursor: None,
+            },
+        )
+        .unwrap();
+    let limited_chars: usize = resp.hits.iter().map(|h| h.snippet.chars().count()).sum();
+
+    assert!(resp.truncated, "small budget must trip truncation");
+    assert!(limited_chars < unrestricted_chars, "snippet should shrink");
+    assert!(!resp.hits.is_empty(), "always retain ≥1 hit");
+}
+
+#[test]
+fn cursor_paginates_to_next_page() {
+    let env = common::TestEnv::new();
+    for i in 0..6 {
+        common::ingest_md(&env, &format!("d{i}.md"), &format!("# T{i}\n\nrust topic {i}\n"));
+    }
+    let app = env.app();
+
+    let page1 = app
+        .search_with_opts(lex("rust", 2), SearchOpts::default())
+        .unwrap();
+    assert_eq!(page1.hits.len(), 2);
+    let cursor = page1.next_cursor.expect("more hits available");
+
+    let page2 = app
+        .search_with_opts(
+            lex("rust", 2),
+            SearchOpts {
+                max_tokens: None,
+                snippet_chars: None,
+                cursor: Some(cursor),
+            },
+        )
+        .unwrap();
+    assert_eq!(page2.hits.len(), 2);
+    let p1_ids: std::collections::HashSet<_> =
+        page1.hits.iter().map(|h| h.chunk_id.0.clone()).collect();
+    let p2_ids: std::collections::HashSet<_> =
+        page2.hits.iter().map(|h| h.chunk_id.0.clone()).collect();
+    assert!(p1_ids.is_disjoint(&p2_ids), "page 2 must not repeat page 1 hits");
+}
+
+#[test]
+fn cursor_rejected_after_corpus_revision_bump() {
+    let env = common::TestEnv::new();
+    common::ingest_md(&env, "a.md", "# T\n\napples\n");
+    let app = env.app();
+
+    let page1 = app
+        .search_with_opts(lex("apples", 1), SearchOpts::default())
+        .unwrap();
+    let cursor = page1.next_cursor;
+
+    if let Some(c) = cursor {
+        common::ingest_md(&env, "b.md", "# B\n\nbananas\n");
+        let app2 = env.app();
+
+        let result = app2.search_with_opts(
+            lex("apples", 1),
+            SearchOpts {
+                max_tokens: None,
+                snippet_chars: None,
+                cursor: Some(c),
+            },
+        );
+        let err = result.unwrap_err();
+        assert!(
+            err.to_string().contains("stale_cursor"),
+            "must surface stale_cursor: {err}"
+        );
+    }
+}