Use MKL for Windows Python 3.12 in test matrix

Diversify BLAS backend testing on Windows:
- Windows + Python 3.12: MKL (Intel optimized)
- Windows + Python 3.13: OpenBLAS (default)

This gives better test coverage across BLAS implementations:
- Linux: OpenBLAS (apt) + MKL (conda)
- macOS: Accelerate framework
- Windows: MKL + OpenBLAS

Author: ilayn

.github/workflows/test-matrix.yml

@@ -34,8 +34,14 @@ jobs:
       run: |
         conda install -y meson ninja numpy pytest openblas pkg-config
 
-    - name: Install dependencies (Windows)
-      if: runner.os == 'Windows'
+    - name: Install dependencies (Windows - Python 3.12 with MKL)
+      if: runner.os == 'Windows' && matrix.python-version == '3.12'
+      shell: bash -l {0}
+      run: |
+        conda install -y meson ninja numpy pytest mkl mkl-devel clang llvm-tools lld pkg-config
+
+    - name: Install dependencies (Windows - Python 3.13 with OpenBLAS)
+      if: runner.os == 'Windows' && matrix.python-version == '3.13'
       shell: bash -l {0}
       run: |
         conda install -y meson ninja numpy pytest openblas clang llvm-tools lld pkg-config