Add an additive, non-required coverage job to CI that runs cargo
llvm-cov and uploads lcov to Codecov (tokenless, fail_ci_if_error
false). Mirrors the test job's environment and pinned 1.90.0 toolchain;
--locked is omitted to match this crate's gitignored-lockfile policy.
Adds codecov.yml (informational, comment off) and a README badge.
