ci: add windows-latest to the test matrix

Guards against Windows-specific regressions like the path-separator
and npm.cmd issues fixed in the preceding commit. Prior CI ran only
on ubuntu-latest, so the test failures reported in this PR went
undetected on main.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -8,9 +8,11 @@ on:
 
 jobs:
   build-and-test:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     strategy:
+      fail-fast: false
       matrix:
+        os: [ubuntu-latest, windows-latest]
         node-version: [20, 22]
 
     steps: