Case-variant locked/record paths on case-insensitive checkouts (A10 from #920) #953

Follow-up from #920 (process.md and the decisions index). Inferred; Linux CI can't test it.

Problem: on a case-insensitive checkout (macOS, Windows), a path like Docs/adr/x.md or Decisions.md is L0 for review levels, which compare paths case-sensitively, but the filesystem can resolve it as a record path that the planners' decisions index reads. A one-review docs PR could then add a citable decision record (inferred).

Fix by design: if #920 round 7 makes the index read only git-tracked blobs at HEAD (ls-tree), the index sees git's exact-case paths, which closes the index side. Then also make review levels compare locked and record paths case-insensitively, failing closed, so a case variant of a locked path is L3 everywhere. Add a test with case-variant paths.

Critical set (review levels).