feat: Complete pipeline hot-rebuild implementation with acceptance tests

- Implements pipeline hot-rebuild with state preservation (issue #43)
- Adds auto-injection of MVP stages for missing capabilities
- Adds radial camera mode for polar coordinate scanning
- Adds afterimage and motionblur effects using framebuffer history
- Adds comprehensive acceptance tests for camera modes and pipeline rebuild
- Updates presets.toml with new effect configurations

Related to: #35 (Pipeline Mutation API epic)
Closes: #43, #44, #45

engine/pipeline/core.py

@@ -155,6 +155,21 @@ class Stage(ABC):
         """
         return set()
 
+    @property
+    def stage_dependencies(self) -> set[str]:
+        """Return set of stage names this stage must connect to directly.
+
+        This allows explicit stage-to-stage dependencies, useful for enforcing
+        pipeline structure when capability matching alone is insufficient.
+
+        Examples:
+            - {"viewport_filter"}  # Must connect to viewport_filter stage
+            - {"camera_update"}    # Must connect to camera_update stage
+
+        NOTE: These are stage names (as added to pipeline), not capabilities.
+        """
+        return set()
+
     def init(self, ctx: "PipelineContext") -> bool:
         """Initialize stage with pipeline context.