fix: resolve cascade correlation, update KNOWN_ISSUES

- MitmStore: added active_cascade_id field with set/get/clear methods
- record_usage() now falls back to active_cascade_id when the heuristic
  cascade hint is absent (fixes usage always going to _latest)
- All three API handlers set active cascade before send_message
- KNOWN_ISSUES: moved 3 issues to resolved:
  - Request modification (already true, was stale entry)
  - Cascade correlation (fixed via active_cascade_id)
  - Progressive thinking streaming (fixed via MITM bypass)

src/api/responses.rs

@@ -278,6 +278,7 @@ pub(crate) async fn handle_responses(
     };
 
     // Send message
+    state.mitm_store.set_active_cascade(&cascade_id).await;
     match state
         .backend
         .send_message(&cascade_id, &user_text, model.model_enum)