feat: add dog effect

- Ignores input, returns dog.mkv from immutable URL
- Downloads from art-source with hash verification
- Caches downloaded file locally
- 5 automated tests (all passing)

Owner: @giles@artdag.rose-ash.com

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

dog/effect.py

@@ -0,0 +1,84 @@
+"""
+Dog effect - returns dog.mkv regardless of input.
+
+This is a constant effect that fetches from an immutable URL.
+"""
+
+import hashlib
+import logging
+import shutil
+from pathlib import Path
+from typing import Any, Dict
+
+import requests
+
+logger = logging.getLogger(__name__)
+
+# Immutable source URL (commit-pinned)
+DOG_URL = "https://git.rose-ash.com/art-dag/art-source/raw/3cb70f9aab6c5e9473e5ce34c59fb1d7dca4bdaa/dog.mkv"
+DOG_HASH = "772f26f9b4e80984788bc48f7c6eee0a1974966b2d4ee56a72d7c6586b3ac9d8"
+
+
+def file_hash(path: Path) -> str:
+    """Compute SHA3-256 hash of a file."""
+    hasher = hashlib.sha3_256()
+    with open(path, "rb") as f:
+        for chunk in iter(lambda: f.read(65536), b""):
+            hasher.update(chunk)
+    return hasher.hexdigest()
+
+
+def effect_dog(input_path: Path, output_path: Path, config: Dict[str, Any]) -> Path:
+    """
+    Dog effect - ignores input, returns dog.mkv.
+
+    Downloads from immutable URL and verifies hash.
+    """
+    output_path.parent.mkdir(parents=True, exist_ok=True)
+
+    # Output with correct extension
+    actual_output = output_path.with_suffix(".mkv")
+    if actual_output.exists():
+        actual_output.unlink()
+
+    # Check cache first
+    cache_dir = Path.home() / ".artdag" / "effect_cache"
+    cache_dir.mkdir(parents=True, exist_ok=True)
+    cached_file = cache_dir / f"{DOG_HASH}.mkv"
+
+    if cached_file.exists():
+        # Verify cached file
+        if file_hash(cached_file) == DOG_HASH:
+            logger.debug(f"EFFECT dog: using cached {cached_file}")
+            shutil.copy2(cached_file, actual_output)
+            return actual_output
+        else:
+            # Cache corrupted, remove it
+            cached_file.unlink()
+
+    # Download from immutable URL
+    logger.info(f"EFFECT dog: downloading from {DOG_URL}")
+    response = requests.get(DOG_URL, stream=True)
+    response.raise_for_status()
+
+    # Write to cache
+    with open(cached_file, "wb") as f:
+        for chunk in response.iter_content(chunk_size=65536):
+            f.write(chunk)
+
+    # Verify hash
+    downloaded_hash = file_hash(cached_file)
+    if downloaded_hash != DOG_HASH:
+        cached_file.unlink()
+        raise ValueError(f"Hash mismatch! Expected {DOG_HASH}, got {downloaded_hash}")
+
+    # Copy to output
+    shutil.copy2(cached_file, actual_output)
+    logger.debug(f"EFFECT dog: {input_path.name} -> {actual_output} (input ignored)")
+
+    return actual_output
+
+
+# Export for registration
+effect = effect_dog
+name = "dog"