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)

engine/pipeline/adapters/camera.py

@@ -16,6 +16,58 @@ class CameraStage(Stage):
         self.optional = True
         self._last_frame_time: float | None = None
 
+    def save_state(self) -> dict[str, Any]:
+        """Save camera state for restoration after pipeline rebuild.
+
+        Returns:
+            Dictionary containing camera state that can be restored
+        """
+        return {
+            "x": self._camera.x,
+            "y": self._camera.y,
+            "mode": self._camera.mode.value
+            if hasattr(self._camera.mode, "value")
+            else self._camera.mode,
+            "speed": self._camera.speed,
+            "zoom": self._camera.zoom,
+            "canvas_width": self._camera.canvas_width,
+            "canvas_height": self._camera.canvas_height,
+            "_x_float": getattr(self._camera, "_x_float", 0.0),
+            "_y_float": getattr(self._camera, "_y_float", 0.0),
+            "_time": getattr(self._camera, "_time", 0.0),
+        }
+
+    def restore_state(self, state: dict[str, Any]) -> None:
+        """Restore camera state from saved state.
+
+        Args:
+            state: Dictionary containing camera state from save_state()
+        """
+        from engine.camera import CameraMode
+
+        self._camera.x = state.get("x", 0)
+        self._camera.y = state.get("y", 0)
+
+        # Restore mode - handle both enum value and direct enum
+        mode_value = state.get("mode", 0)
+        if isinstance(mode_value, int):
+            self._camera.mode = CameraMode(mode_value)
+        else:
+            self._camera.mode = mode_value
+
+        self._camera.speed = state.get("speed", 1.0)
+        self._camera.zoom = state.get("zoom", 1.0)
+        self._camera.canvas_width = state.get("canvas_width", 200)
+        self._camera.canvas_height = state.get("canvas_height", 200)
+
+        # Restore internal state
+        if hasattr(self._camera, "_x_float"):
+            self._camera._x_float = state.get("_x_float", 0.0)
+        if hasattr(self._camera, "_y_float"):
+            self._camera._y_float = state.get("_y_float", 0.0)
+        if hasattr(self._camera, "_time"):
+            self._camera._time = state.get("_time", 0.0)
+
     @property
     def stage_type(self) -> str:
         return "camera"

engine/pipeline/adapters/display.py

@@ -13,6 +13,39 @@ class DisplayStage(Stage):
         self.name = name
         self.category = "display"
         self.optional = False
+        self._initialized = False
+        self._init_width = 80
+        self._init_height = 24
+
+    def save_state(self) -> dict[str, Any]:
+        """Save display state for restoration after pipeline rebuild.
+
+        Returns:
+            Dictionary containing display state that can be restored
+        """
+        return {
+            "initialized": self._initialized,
+            "init_width": self._init_width,
+            "init_height": self._init_height,
+            "width": getattr(self._display, "width", 80),
+            "height": getattr(self._display, "height", 24),
+        }
+
+    def restore_state(self, state: dict[str, Any]) -> None:
+        """Restore display state from saved state.
+
+        Args:
+            state: Dictionary containing display state from save_state()
+        """
+        self._initialized = state.get("initialized", False)
+        self._init_width = state.get("init_width", 80)
+        self._init_height = state.get("init_height", 24)
+
+        # Restore display dimensions if the display supports it
+        if hasattr(self._display, "width"):
+            self._display.width = state.get("width", 80)
+        if hasattr(self._display, "height"):
+            self._display.height = state.get("height", 24)
 
     @property
     def capabilities(self) -> set[str]:
@@ -37,7 +70,17 @@ class DisplayStage(Stage):
     def init(self, ctx: PipelineContext) -> bool:
         w = ctx.params.viewport_width if ctx.params else 80
         h = ctx.params.viewport_height if ctx.params else 24
-        result = self._display.init(w, h, reuse=False)
+
+        # Try to reuse display if already initialized
+        reuse = self._initialized
+        result = self._display.init(w, h, reuse=reuse)
+
+        # Update initialization state
+        if result is not False:
+            self._initialized = True
+            self._init_width = w
+            self._init_height = h
+
         return result is not False
 
     def process(self, data: Any, ctx: PipelineContext) -> Any:

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()

engine/pipeline/ui.py

@@ -78,6 +78,58 @@ class UIPanel:
         self._show_panel: bool = True  # UI panel visibility
         self._preset_scroll_offset: int = 0  # Scroll in preset list
 
+    def save_state(self) -> dict[str, Any]:
+        """Save UI panel state for restoration after pipeline rebuild.
+
+        Returns:
+            Dictionary containing UI panel state that can be restored
+        """
+        # Save stage control states (enabled, params, etc.)
+        stage_states = {}
+        for name, ctrl in self.stages.items():
+            stage_states[name] = {
+                "enabled": ctrl.enabled,
+                "selected": ctrl.selected,
+                "params": dict(ctrl.params),  # Copy params dict
+            }
+
+        return {
+            "stage_states": stage_states,
+            "scroll_offset": self.scroll_offset,
+            "selected_stage": self.selected_stage,
+            "_focused_param": self._focused_param,
+            "_show_panel": self._show_panel,
+            "_show_preset_picker": self._show_preset_picker,
+            "_preset_scroll_offset": self._preset_scroll_offset,
+        }
+
+    def restore_state(self, state: dict[str, Any]) -> None:
+        """Restore UI panel state from saved state.
+
+        Args:
+            state: Dictionary containing UI panel state from save_state()
+        """
+        # Restore stage control states
+        stage_states = state.get("stage_states", {})
+        for name, stage_state in stage_states.items():
+            if name in self.stages:
+                ctrl = self.stages[name]
+                ctrl.enabled = stage_state.get("enabled", True)
+                ctrl.selected = stage_state.get("selected", False)
+                # Restore params
+                saved_params = stage_state.get("params", {})
+                for param_name, param_value in saved_params.items():
+                    if param_name in ctrl.params:
+                        ctrl.params[param_name] = param_value
+
+        # Restore UI panel state
+        self.scroll_offset = state.get("scroll_offset", 0)
+        self.selected_stage = state.get("selected_stage")
+        self._focused_param = state.get("_focused_param")
+        self._show_panel = state.get("_show_panel", True)
+        self._show_preset_picker = state.get("_show_preset_picker", False)
+        self._preset_scroll_offset = state.get("_preset_scroll_offset", 0)
+
     def register_stage(self, stage: Any, enabled: bool = True) -> StageControl:
         """Register a stage for UI control.