E8: add code coverage to CI

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.

.github/workflows/ci.yml

@@ -86,6 +86,41 @@ jobs:
       - name: Test on MSRV
         run: cargo test ${{ matrix.packages }} --verbose
 
+  # Code coverage for the pure-Rust crates, uploaded to Codecov.
+  coverage:
+    name: Coverage
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+
+      - name: Install Rust toolchain
+        uses: dtolnay/rust-toolchain@29eef336d9b2848a0b548edc03f92a220660cdb8 # stable branch, 2026-03-27
+        with:
+          components: llvm-tools-preview
+
+      - name: Cache cargo
+        uses: Swatinem/rust-cache@e18b497796c12c097a38f9edb9d0641fb99eee32 # v2
+
+      - name: Install cargo-llvm-cov
+        uses: taiki-e/install-action@e0eafa9a0d485c37f97c0f7beb930a58a2facbac # v2.79.4
+        with:
+          tool: cargo-llvm-cov
+
+      - name: Generate coverage (lcov)
+        run: >
+          cargo llvm-cov
+          -p wickra-core -p wickra -p wickra-data
+          --features wickra-data/live-binance
+          --lcov --output-path lcov.info
+
+      - name: Upload to Codecov
+        uses: codecov/codecov-action@75cd11691c0faa626561e295848008c8a7dddffe # v5.5.4
+        with:
+          files: lcov.info
+          fail_ci_if_error: false
+        env:
+          CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
+
   # Supply-chain audit: security advisories, license policy, banned crates,
   # and source restrictions. Configured by deny.toml at the repo root.
   supply-chain:

README.md

@@ -1,6 +1,7 @@
 # Wickra
 
 [![CI](https://github.com/kingchenc/wickra/actions/workflows/ci.yml/badge.svg)](https://github.com/kingchenc/wickra/actions/workflows/ci.yml)
+[![codecov](https://codecov.io/gh/kingchenc/wickra/branch/main/graph/badge.svg)](https://codecov.io/gh/kingchenc/wickra)
 [![crates.io](https://img.shields.io/crates/v/wickra.svg?logo=rust&color=orange)](https://crates.io/crates/wickra)
 [![PyPI](https://img.shields.io/pypi/v/wickra.svg?logo=pypi&color=blue)](https://pypi.org/project/wickra/)
 [![npm](https://img.shields.io/npm/v/wickra.svg?logo=npm&color=red)](https://www.npmjs.com/package/wickra)