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

tests/test_app.py

@@ -27,13 +27,13 @@ class TestMain:
         """main() uses PRESET from config if set."""
         with (
             patch("engine.config.PIPELINE_DIAGRAM", False),
-            patch("engine.config.PRESET", "gallery-sources"),
+            patch("engine.config.PRESET", "demo"),
             patch("engine.config.PIPELINE_MODE", False),
             patch("engine.app.main.run_pipeline_mode") as mock_run,
         ):
             sys.argv = ["mainline.py"]
             main()
-            mock_run.assert_called_once_with("gallery-sources")
+            mock_run.assert_called_once_with("demo")
 
     def test_main_exits_on_unknown_preset(self):
         """main() exits with error for unknown preset."""
@@ -122,7 +122,7 @@ class TestRunPipelineMode:
             mock_create.return_value = mock_display
 
             try:
-                run_pipeline_mode("gallery-display-terminal")
+                run_pipeline_mode("demo-base")
             except (KeyboardInterrupt, SystemExit):
                 pass