feat(app): cursor encode/decode for paginated search (fb-34)

Opaque base64(JSON{offset, corpus_revision}). Mismatch or
malformed input returns ErrorV1 with code = stale_cursor.
base64 promoted to workspace dep.

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

crates/kebab-app/tests/cursor.rs

@@ -0,0 +1,24 @@
+//! p9-fb-34: cursor encode/decode round-trip + corpus_revision mismatch.
+
+use kebab_app::cursor;
+
+#[test]
+fn cursor_roundtrip_preserves_offset() {
+    let encoded = cursor::encode(5, "rev-abc");
+    let offset = cursor::decode(&encoded, "rev-abc").unwrap();
+    assert_eq!(offset, 5);
+}
+
+#[test]
+fn cursor_decode_rejects_mismatched_revision() {
+    let encoded = cursor::encode(7, "rev-old");
+    let err = cursor::decode(&encoded, "rev-new").unwrap_err();
+    assert_eq!(err.code, "stale_cursor");
+    assert!(err.message.contains("rev-old") || err.message.contains("rev-new"));
+}
+
+#[test]
+fn cursor_decode_rejects_garbage_input() {
+    let err = cursor::decode("not-base64!!!", "any").unwrap_err();
+    assert_eq!(err.code, "stale_cursor");
+}