Fix CI: add rlib crate-type and use maturin build

- Add rlib to crate-type so cargo test can compile the library
- Replace maturin-action develop with maturin build + pip install
  (develop command requires virtualenv which isn't set up)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: igerber

.github/workflows/rust-test.yml

@@ -74,15 +74,15 @@ jobs:
       - name: Install Rust toolchain
         uses: dtolnay/rust-toolchain@stable
 
-      - name: Build with maturin
-        uses: PyO3/maturin-action@v1
-        with:
-          command: develop
-          args: --release
-
       - name: Install test dependencies
         run: pip install pytest numpy pandas scipy
 
+      - name: Build and install with maturin
+        run: |
+          pip install maturin
+          maturin build --release
+          pip install target/wheels/*.whl
+
       - name: Verify Rust backend is available
         run: |
           python -c "from diff_diff import HAS_RUST_BACKEND; assert HAS_RUST_BACKEND, 'Rust backend not available'"

rust/Cargo.toml

@@ -7,7 +7,8 @@ license = "MIT"
 
 [lib]
 name = "diff_diff_rust"
-crate-type = ["cdylib"]
+# cdylib for Python extension, rlib for running tests
+crate-type = ["cdylib", "rlib"]
 
 [dependencies]
 pyo3 = { version = "0.20", features = ["extension-module"] }