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>

dog/effect.py

@@ -1,14 +1,27 @@
+# /// script
+# requires-python = ">=3.10"
+# dependencies = ["requests"]
+# ///
 """
-Dog effect - returns dog.mkv regardless of input.
+@effect dog
+@version 1.0.0
+@author @giles@artdag.rose-ash.com
+@temporal true
 
-This is a constant effect that fetches from an immutable URL.
+@description
+Returns dog.mkv regardless of input. This is a constant effect that
+fetches from an immutable URL and verifies the content hash.
+Demonstrates a whole-video effect that ignores its input.
+
+@example
+  (fx dog)
 """
 
 import hashlib
 import logging
 import shutil
 from pathlib import Path
-from typing import Any, Dict
+from typing import Any, Dict, List
 
 import requests
 
@@ -28,12 +41,13 @@ def file_hash(path: Path) -> str:
     return hasher.hexdigest()
 
 
-def effect_dog(input_path: Path, output_path: Path, config: Dict[str, Any]) -> Path:
+def process(input_paths: List[Path], output_path: Path, params: Dict[str, Any], ctx) -> Path:
     """
-    Dog effect - ignores input, returns dog.mkv.
+    Whole-video API: ignores input, returns dog.mkv.
 
     Downloads from immutable URL and verifies hash.
     """
+    output_path = Path(output_path)
     output_path.parent.mkdir(parents=True, exist_ok=True)
 
     # Output with correct extension
@@ -74,11 +88,6 @@ def effect_dog(input_path: Path, output_path: Path, config: Dict[str, Any]) -> P
 
     # Copy to output
     shutil.copy2(cached_file, actual_output)
-    logger.debug(f"EFFECT dog: {input_path.name} -> {actual_output} (input ignored)")
+    logger.debug(f"EFFECT dog: -> {actual_output} (input ignored)")
 
     return actual_output
-
-
-# Export for registration
-effect = effect_dog
-name = "dog"

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