CI had no coverage measurement, and although .gitignore listed coverage
artefacts nothing produced them.
Add a `coverage` job that runs cargo-llvm-cov over the three pure-Rust
crates (with wickra-data's live-binance feature so the Binance parser
tests count), emits lcov, and uploads to Codecov. The Codecov upload
uses fail_ci_if_error: false so a Codecov outage cannot break CI. Both
new actions (taiki-e/install-action, codecov/codecov-action) are
SHA-pinned. Add a coverage badge to the README.
kingchenc
5568596e18