ci: expand test matrix to macOS and Windows

unpythonic is pure Python but exercises unusual control-flow
machinery (continuations, dynvars, TCO, generator tricks), where
OS-specific regressions can surface independently of the Python
minor version.

Matrix shape: full Python 3.10–3.14 plus pypy-3.11 on Linux
(where most contributors develop); newest CPython (3.14) plus
pypy-3.11 on macOS and Windows.  PyPy is included on every OS
because it's a separate interpreter family — the control-flow
code paths differ from CPython and can have their own quirks.

To make the workflow cross-platform:

- Add a job-level `shell: bash` default so the `tr` used in
  "Determine Python version string for PDM" works on Windows
  (uses Git Bash).
- Replace the `pdm use --venv in-project && source .venv/bin/activate
  && python runtests.py` activation dance in the test step with
  `pdm run python runtests.py`.  `source .venv/bin/activate` hard-
  codes the POSIX venv layout and would fail on Windows (where the
  activate script lives at .venv/Scripts/activate).  `pdm run`
  abstracts the platform-specific layout and uses the same venv
  that the earlier `pdm install` step created — matching the
  convention already documented in CLAUDE.md.

Also add `fail-fast: false` so a failure in one cell doesn't
cancel the others: the diagnostic value of seeing which
(Python × OS) combinations regress outweighs the minute savings
from aborting early, especially with a matrix this small.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Technologicat

.github/workflows/ci.yml

@@ -33,10 +33,31 @@ jobs:
 
   test:
     needs: lint
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
+    defaults:
+      run:
+        shell: bash  # so the `tr` in "Determine Python version string for PDM" works on Windows (uses Git Bash)
     strategy:
+      fail-fast: false
       matrix:
+        # Full Python/PyPy matrix on Linux; newest CPython plus PyPy on
+        # macOS and Windows.  Rationale: unpythonic is pure Python but
+        # exercises unusual control-flow machinery (continuations, dynvars,
+        # TCO, generator tricks), where OS-specific regressions can surface
+        # independently of Python minor version.  PyPy is included on every
+        # OS because it's a separate interpreter family — the control-flow
+        # code paths differ from CPython and can have their own quirks.
+        os: [ubuntu-latest]
         python-version: ["3.10", "3.11", "3.12", "3.13", "3.14", "pypy-3.11"]
+        include:
+          - os: macos-latest
+            python-version: "3.14"
+          - os: macos-latest
+            python-version: "pypy-3.11"
+          - os: windows-latest
+            python-version: "3.14"
+          - os: windows-latest
+            python-version: "pypy-3.11"
 
     steps:
     - uses: actions/checkout@v6
@@ -62,10 +83,7 @@ jobs:
         #     https://pdm-project.org/en/latest/usage/venv/
         pdm install
     - name: Test with unpythonic.test.fixtures
-      run: |
-        pdm use --venv in-project
-        source .venv/bin/activate
-        python runtests.py
+      run: pdm run python runtests.py
 
   build-dist:
     needs: test