docs: correct the LFS trap and add Tools/check-lfs.sh

The lfs filter is inactive until 'git lfs install' writes filter.lfs.*, and a
matched file is then stored as a plain blob without any warning. Document that
instead of the wrong claim that git add fails, and add a guard script.

Tools/README.md

@@ -12,6 +12,7 @@ Repeatable import/export and processing helpers. Anything that was done twice by
 
 ## Planned helpers
 
+- `check-lfs.sh` — repository guard (exists): fails if `git-lfs` or the `lfs` filter is missing, or if an LFS-tracked file was committed as a plain blob
 - Blender export preset runner (named presets, metric scale assertion)
 - Blender→Unreal scale test (export a known-size cube, verify 100× conversion)
 - OSM extract fetch and manifest writer (records date, bounds, checksum)