chore/refactor-ci-workflows (#6)

* ci(workflows): split CI into focused workflows and add Codecov upload

* ci(coverage): upload to codecov for same-repo PRs and main pushes

* fix(ci): upload coverage to codecov only on main pushes

.github/workflows/coverage.yml

@@ -0,0 +1,27 @@
+name: Coverage
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  coverage:
+    name: cargo llvm-cov + codecov
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          components: llvm-tools-preview
+      - uses: Swatinem/rust-cache@v2
+      - uses: taiki-e/install-action@cargo-llvm-cov
+      - run: cargo llvm-cov --workspace --all-targets --lcov --output-path lcov.info
+      - name: Upload coverage reports to Codecov
+        uses: codecov/codecov-action@v5
+        if: github.event_name == 'push' && github.ref == 'refs/heads/main'
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}
+          files: lcov.info
+          fail_ci_if_error: true