ci: implement comprehensive GitHub Actions workflow

Replace placeholder CI workflow with production-ready pipeline implementing automated testing, linting, and multi-platform builds.

Changes:
  - Add test job running cargo test on Ubuntu with cargo caching
  - Add fmt job for rustfmt code formatting checks
  - Add clippy job for lint checks (treats warnings as errors)
  - Add build job with matrix strategy for Ubuntu, macOS, and Windows
  - Configure triggers for push/PR to main branch + manual dispatch
  - Enable cargo caching for registry, index, and build artifacts
  - Set CARGO_TERM_COLOR and RUST_BACKTRACE environment variables

Author: HendrikReh

.github/workflows/ci.yml

@@ -1,10 +1,126 @@
-name: CI (placeholder)
+name: CI
 
 on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
   workflow_dispatch:
 
+env:
+  CARGO_TERM_COLOR: always
+  RUST_BACKTRACE: 1
+
 jobs:
-  todo:
+  test:
+    name: Test
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Install Rust toolchain
+        uses: dtolnay/rust-toolchain@stable
+
+      - name: Cache cargo registry
+        uses: actions/cache@v4
+        with:
+          path: ~/.cargo/registry
+          key: ${{ runner.os }}-cargo-registry-${{ hashFiles('**/Cargo.lock') }}
+
+      - name: Cache cargo index
+        uses: actions/cache@v4
+        with:
+          path: ~/.cargo/git
+          key: ${{ runner.os }}-cargo-index-${{ hashFiles('**/Cargo.lock') }}
+
+      - name: Cache cargo build
+        uses: actions/cache@v4
+        with:
+          path: target
+          key: ${{ runner.os }}-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}
+
+      - name: Run tests
+        run: cargo test --all-features --workspace
+
+  fmt:
+    name: Format
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Install Rust toolchain
+        uses: dtolnay/rust-toolchain@stable
+        with:
+          components: rustfmt
+
+      - name: Check formatting
+        run: cargo fmt --all -- --check
+
+  clippy:
+    name: Clippy
     runs-on: ubuntu-latest
     steps:
-      - run: echo "TODO: configure fmt, lint, test, deny, audit pipelines"
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Install Rust toolchain
+        uses: dtolnay/rust-toolchain@stable
+        with:
+          components: clippy
+
+      - name: Cache cargo registry
+        uses: actions/cache@v4
+        with:
+          path: ~/.cargo/registry
+          key: ${{ runner.os }}-cargo-registry-${{ hashFiles('**/Cargo.lock') }}
+
+      - name: Cache cargo index
+        uses: actions/cache@v4
+        with:
+          path: ~/.cargo/git
+          key: ${{ runner.os }}-cargo-index-${{ hashFiles('**/Cargo.lock') }}
+
+      - name: Cache cargo build
+        uses: actions/cache@v4
+        with:
+          path: target
+          key: ${{ runner.os }}-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}
+
+      - name: Run clippy
+        run: cargo clippy --all-targets --all-features --workspace -- -D warnings
+
+  build:
+    name: Build
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Install Rust toolchain
+        uses: dtolnay/rust-toolchain@stable
+
+      - name: Cache cargo registry
+        uses: actions/cache@v4
+        with:
+          path: ~/.cargo/registry
+          key: ${{ runner.os }}-cargo-registry-${{ hashFiles('**/Cargo.lock') }}
+
+      - name: Cache cargo index
+        uses: actions/cache@v4
+        with:
+          path: ~/.cargo/git
+          key: ${{ runner.os }}-cargo-index-${{ hashFiles('**/Cargo.lock') }}
+
+      - name: Cache cargo build
+        uses: actions/cache@v4
+        with:
+          path: target
+          key: ${{ runner.os }}-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}
+
+      - name: Build release binary
+        run: cargo build --release --all-features --workspace