Finish the image-root task and untrack Blender sources

- ignore assets/models/blender_src/*.blend (keep the .gdignore marker tracked) and
  remove the 46 editable .blend files from the index; they remain on disk and are
  regenerated by tools/blender_assets.py
- move the completed storage/docs/code-path/art-loader/verification, mouse-fix,
  and pop-up sections to DONE.md; TODO now holds only the open lighting item,
  realism follow-ups, and optional ideas
- docs changes only; runtime code unchanged

.gitignore

@@ -64,3 +64,5 @@ docs/blender_previews/*.png
 assets/models/blender/**/*.png
 !assets/models/blender/**/*_ao.png
 assets/models/blender_src/*.blend1
+# Editable Blender sources are developer-only; keep the .gdignore marker tracked.
+assets/models/blender_src/*.blend