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/validation.py

@@ -22,6 +22,8 @@ VALID_CAMERAS = [
     "omni",
     "floating",
     "bounce",
+    "radial",
+    "static",
     "none",
     "",
 ]
@@ -43,7 +45,7 @@ class ValidationResult:
 MVP_DEFAULTS = {
     "source": "fixture",
     "display": "terminal",
-    "camera": "",  # Empty = no camera stage (static viewport)
+    "camera": "static",  # Static camera provides camera_y=0 for viewport filtering
     "effects": [],
     "border": False,
 }