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/fmt.yml

@@ -0,0 +1,19 @@
+name: Rust Format
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  cargo-fmt:
+    name: cargo fmt --check
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          components: rustfmt
+      - uses: Swatinem/rust-cache@v2
+      - run: cargo fmt --check