CI: fix the Install step (setup-uv already creates .venv) and make sure a C linker exists for the Rust build

.github/workflows/ci.yml

@@ -16,7 +16,8 @@ jobs:
         with:
           python-version: ${{ matrix.python }}
       - name: Install
-        run: uv venv && uv pip install -e ".[dev]"
+        # setup-uv >= 5.x already creates and activates .venv when python-version is set; --clear works either way
+        run: uv venv --clear && uv pip install -e ".[dev]"
       - name: Lint
         run: uv run ruff check src tests && uv run ruff format --check src tests
       - name: Type check
@@ -25,6 +26,7 @@ jobs:
         run: uv run pytest tests/unit -q
       - name: Build the compiled kernels (rust/simurg-core)
         run: |
+          command -v cc >/dev/null || { apt-get update -qq && apt-get install -y -qq gcc >/dev/null; }  # Rust needs a C linker
           curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --profile minimal
           PATH="$HOME/.cargo/bin:$PATH" uv pip install ./rust/simurg-core
       - name: Unit tests with the compiled kernels (must equal the pure-Python results)