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/README.md

@@ -0,0 +1,33 @@
+# Dog Effect
+
+The dog effect ignores its input and returns the dog video.
+
+## Signature
+
+```
+dog(input) → dog.mkv
+```
+
+For any input X: `dog(X) = dog.mkv`
+
+## Source
+
+Fetches from immutable URL:
+```
+https://git.rose-ash.com/art-dag/art-source/raw/3cb70f9aab6c5e9473e5ce34c59fb1d7dca4bdaa/dog.mkv
+```
+
+## Expected Output Hash
+
+```
+772f26f9b4e80984788bc48f7c6eee0a1974966b2d4ee56a72d7c6586b3ac9d8
+```
+
+## Properties
+
+- **Constant**: Output is independent of input
+- **Idempotent**: `dog(dog(x)) = dog(x)`
+
+## Owner
+
+`@giles@artdag.rose-ash.com`