ci: use host py launcher for Windows tests (runner blocks setup-python)

The self-hosted Windows runner's PowerShell execution policy rejects
setup-python's install script, so Windows mirrors release.yml: py -3.12
+ venv (the version the release binaries ship with). Linux keeps the
full 3.10/3.12/3.13 setup-python matrix.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -34,17 +34,32 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, windows-latest]
+        os: [ubuntu-latest]
         python: ["3.10", "3.12", "3.13"]
+        include:
+          # Windows tests on 3.12 only — the version the release binaries ship
+          # with. The self-hosted Windows runner blocks setup-python's install
+          # script (PowerShell execution policy), so it uses the host's `py`
+          # launcher + venv, same as release.yml.
+          - os: windows-latest
+            python: "3.12"
     steps:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Set up Python ${{ matrix.python }}
+      - name: Set up Python ${{ matrix.python }} (Linux)
+        if: runner.os == 'Linux'
         uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.python }}
 
+      - name: Set up Python venv (Windows)
+        if: runner.os == 'Windows'
+        shell: pwsh
+        run: |
+          py -${{ matrix.python }} -m venv .venv
+          Add-Content -Path $env:GITHUB_PATH -Value "$env:GITHUB_WORKSPACE\.venv\Scripts"
+
       # bcc_core has no GUI imports, so the test suite needs no PySide6 —
       # keeps CI fast and avoids Qt system-library headaches on the runner.
       - name: Install test dependencies