README development commands show a coverage floor of 89 while CI enforces 94

[ElbertePlinio]

Found during review of #42 (issue #41), and present on main at 4de9c09. #42 does not change it.

The development section of README.md lists cargo llvm-cov --workspace --locked --fail-under-lines 89, but .github/workflows/ci.yml runs the same command with --fail-under-lines 94. A contributor who runs the README command locally can pass at 90% and then fail CI.

Acceptance

• The README command uses the same floor as CI.