feat: disk caching for GIF frames and banners, debug focus handler

- Persist animated GIF frames to user://boxart/itch/anim_frames/<hash>/
  so subsequent loads skip ffmpeg extraction entirely.
- Persist static GIF first-frame PNGs alongside for the same reason.
- Cache stitched banner images via OGPU Cache with a synthetic key
  from both screenshot URLs, avoiding re-stitch on every load.
- Add comprehensive debug logging to _on_focus_changed to diagnose
  why AnimatedTextures aren't playing (logs control name, type,
  GameCard ancestor, texture type).
- Fix plugin.json version to match git tag (was stuck at 0.1.14).

Bump to v0.1.18

plugin.json

@@ -1,7 +1,7 @@
 {
   "plugin.id": "itch-artprovider",
   "plugin.name": "itch.io Art Provider",
-  "plugin.version": "0.1.14",
+  "plugin.version": "0.1.18",
   "plugin.min-api-version": "1.1.0",
   "plugin.link": "https://forge.thergic.ar/jose/itchio-opengamepadui-artprovider-plugin",
   "plugin.source": "https://forge.thergic.ar/jose/itchio-opengamepadui-artprovider-plugin",