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/pipeline/controller.py

@@ -204,6 +204,15 @@ class Pipeline:
         if hasattr(old_stage, "_enabled"):
             new_stage._enabled = old_stage._enabled
 
+        # Preserve camera state
+        if hasattr(old_stage, "save_state") and hasattr(new_stage, "restore_state"):
+            try:
+                state = old_stage.save_state()
+                new_stage.restore_state(state)
+            except Exception:
+                # If state preservation fails, continue without it
+                pass
+
     def _rebuild(self) -> None:
         """Rebuild execution order after mutation without full reinitialization."""
         self._capability_map = self._build_capability_map()