Implement pipeline hot-rebuild with state preservation

- Add save_state/restore_state methods to CameraStage
- Add save_state/restore_state methods to DisplayStage
- Extend Pipeline._copy_stage_state() to preserve camera/display state
- Add save_state/restore_state methods to UIPanel for UI state preservation
- Update pipeline_runner to preserve UI state across preset changes

Camera state preserved:
- Position (x, y)
- Mode (feed, scroll, horizontal, etc.)
- Speed, zoom, canvas dimensions
- Internal timing state

Display state preserved:
- Initialization status
- Dimensions
- Reuse flag for display reinitialization

UI Panel state preserved:
- Stage enabled/disabled status
- Parameter values
- Selected stage and focused parameter
- Scroll position

This enables manual/event-driven rebuilds when inlet-outlet connections change,
while preserving all relevant state across pipeline mutations.

engine/app/pipeline_runner.py

@@ -349,6 +349,9 @@ def run_pipeline_mode(preset_name: str = "demo"):
 
             print(f"  \033[38;5;245mSwitching to preset: {preset_name}\033[0m")
 
+            # Save current UI panel state before rebuild
+            ui_state = ui_panel.save_state() if ui_panel else None
+
             try:
                 # Clean up old pipeline
                 pipeline.cleanup()
@@ -558,6 +561,10 @@ def run_pipeline_mode(preset_name: str = "demo"):
                             )
                             stage_control.effect = effect  # type: ignore[attr-defined]
 
+                    # Restore UI panel state if it was saved
+                    if ui_state:
+                        ui_panel.restore_state(ui_state)
+
                     if ui_panel.stages:
                         first_stage = next(iter(ui_panel.stages))
                         ui_panel.select_stage(first_stage)