feat(diff): Add diff rendering helpers for GUI

- Add get_line_color() to classify diff lines
- Add render_diff_text_immediate() for immediate mode rendering
- Add tests for rendering functions

conductor/tracks.md

@@ -26,7 +26,7 @@ This file tracks all major tracks for the project. Each track has its own detail
 3. [x] **Track: Visual DAG & Interactive Ticket Editing**
    *Link: [./tracks/visual_dag_ticket_editing_20260306/](./tracks/visual_dag_ticket_editing_20260306/)*
 
-4. [ ] **Track: Advanced Tier 4 QA Auto-Patching**
+4. [~] **Track: Advanced Tier 4 QA Auto-Patching**
    *Link: [./tracks/tier4_auto_patching_20260306/](./tracks/tier4_auto_patching_20260306/)*
 
 5. [ ] **Track: Transitioning to Native Orchestrator**