feat: add test coverage reporting

.github/workflows/rust.yml

@@ -48,6 +48,42 @@ jobs:
       - name: Run cargo test
         run: cargo test --verbose --workspace
 
+  coverage:
+    name: coverage (llvm-cov)
+    runs-on: ubuntu-latest
+    needs: build
+    env:
+      COVERAGE_LINES_MIN: "80"
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Rust 1.90 + llvm-tools
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: "1.90"
+          override: true
+          components: llvm-tools-preview
+
+      - name: Install cargo-llvm-cov
+        uses: taiki-e/install-action@cargo-llvm-cov
+        with:
+          version: "0.6.12"
+
+      - name: Rust Cache
+        uses: Swatinem/rust-cache@v2
+        with:
+          key: ${{ runner.os }}-${{ runner.arch }}-cargo-${{ hashFiles('**/Cargo.toml') }}-coverage
+          cache-on-failure: false
+
+      - name: Run coverage
+        run: cargo llvm-cov --workspace --lcov --output-path lcov.info --fail-under-lines "$COVERAGE_LINES_MIN"
+
+      - name: Upload coverage report
+        uses: actions/upload-artifact@v4
+        with:
+          name: coverage-lcov
+          path: lcov.info
+
   e2e-s3:
     name: e2e (s3 via LocalStack)
     runs-on: ubuntu-latest

.gitignore

@@ -12,4 +12,6 @@ __pycache__
 node_modules
 dist
 
+lcov.info
+
 .DS_Store

README.md

@@ -110,6 +110,29 @@ See [docs/overview.md](./docs/overview.md) for the full design.
 - **API Reference**: [docs.rs/tonbo](https://docs.rs/tonbo): full Rust API documentation
 - **RFCs**: [docs/rfcs/](./docs/rfcs/): design documents for contributors
 
+## Development
+
+### Coverage
+
+Install coverage tooling once:
+
+```bash
+rustup component add llvm-tools-preview
+cargo install cargo-llvm-cov --version 0.6.12 --locked
+```
+
+Run coverage locally:
+
+```bash
+cargo llvm-cov --workspace --lcov --output-path lcov.info --summary
+```
+
+Generate an HTML report:
+
+```bash
+cargo llvm-cov --workspace --html
+```
+
 ### Project status
 
 Tonbo is currently in **alpha**. APIs may change, and we're actively iterating based on feedback.