Convert effects to new single-file format

- dog: Convert to whole-video API with PEP 723 metadata
- identity: Add as frame-by-frame effect

Both now use @-tag docstrings for AI-readable metadata.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

identity/effect.py

@@ -0,0 +1,35 @@
+# /// script
+# requires-python = ">=3.10"
+# dependencies = []
+# ///
+"""
+@effect identity
+@version 1.0.0
+@author @giles@artdag.rose-ash.com
+@temporal false
+
+@description
+Identity effect - returns input unchanged. This is the foundational
+effect where identity(x) = x. Uses frame-by-frame API but simply
+passes frames through unmodified.
+
+@example
+  (fx identity)
+"""
+
+import numpy as np
+
+
+def process_frame(frame: np.ndarray, params: dict, state) -> tuple:
+    """
+    Identity: return frame unchanged.
+
+    Args:
+        frame: RGB frame as numpy array (H, W, 3)
+        params: Unused
+        state: Passed through unchanged
+
+    Returns:
+        (frame, state) - both unchanged
+    """
+    return frame, state