Linux is case-sensitive; Windows and macOS are not. Two tracked paths
differing only by case (README.md vs readme.md, src/Foo.py vs SRC/foo.py)
land fine on Linux and silently break every clone on a case-insensitive
host — the filesystem holds one, so checkout fails or whichever wins
clobbers the other. Git won't stop the pair from landing; it only warns
at checkout time on a case-insensitive FS. This is the enforcement point.
Adds scripts/check-case-collisions.py (index scan keyed on casefolded
full paths) + an unconditional workflow_call job wired into ci.yaml and
the all-checks-pass gate — unconditional because a collision can ship in
any kind of PR (docs, JS, config), not just Python, so gating on a
language lane would be the same passive-rule trap the infographic check
closes. Tests in tests/scripts/test_case_collision_check.py build
collisions via git update-index --cacheinfo so they run on
case-insensitive filesystems too.
