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/adapters/transform.py

@@ -49,7 +49,9 @@ class ViewportFilterStage(Stage):
 
     @property
     def dependencies(self) -> set[str]:
-        return {"source"}
+        # Always requires camera.state for viewport filtering
+        # CameraUpdateStage provides this (auto-injected if missing)
+        return {"source", "camera.state"}
 
     @property
     def inlet_types(self) -> set:
@@ -95,9 +97,13 @@ class ViewportFilterStage(Stage):
 
         # Find start index (first item that intersects with visible range)
         start_idx = 0
+        start_item_y = 0  # Y position where the first visible item starts
         for i, total_h in enumerate(cumulative_heights):
             if total_h > start_y:
                 start_idx = i
+                # Calculate the Y position of the start of this item
+                if i > 0:
+                    start_item_y = cumulative_heights[i - 1]
                 break
 
         # Find end index (first item that extends beyond visible range)
@@ -107,6 +113,16 @@ class ViewportFilterStage(Stage):
                 end_idx = i + 1
                 break
 
+        # Adjust camera_y for the filtered buffer
+        # The filtered buffer starts at row 0, but the camera position
+        # needs to be relative to where the first visible item starts
+        filtered_camera_y = camera_y - start_item_y
+
+        # Update context with the filtered camera position
+        # This ensures CameraStage can correctly slice the filtered buffer
+        ctx.set_state("camera_y", filtered_camera_y)
+        ctx.set_state("camera_x", ctx.get("camera_x", 0))  # Keep camera_x unchanged
+
         # Return visible items
         return data[start_idx:end_idx]
 
@@ -127,9 +143,16 @@ class FontStage(Stage):
     def capabilities(self) -> set[str]:
         return {"render.output"}
 
+    @property
+    def stage_dependencies(self) -> set[str]:
+        # Must connect to viewport_filter stage to get filtered source
+        return {"viewport_filter"}
+
     @property
     def dependencies(self) -> set[str]:
-        return {"source"}
+        # Depend on source.filtered (provided by viewport_filter)
+        # This ensures we get the filtered/processed source, not raw source
+        return {"source.filtered"}
 
     @property
     def inlet_types(self) -> set:
@@ -147,6 +170,11 @@ class FontStage(Stage):
         if not isinstance(data, list):
             return [str(data)]
 
+        import os
+
+        if os.environ.get("DEBUG_CAMERA"):
+            print(f"FontStage: input items={len(data)}")
+
         viewport_width = ctx.params.viewport_width if ctx.params else 80
 
         result = []