ci: run verify/build on Linux and Windows, allow manual dispatch

Add a fail-fast: false OS matrix (ubuntu-latest, windows-latest) and a
workflow_dispatch trigger so CI can be run on demand.

.github/workflows/ci.yml

@@ -4,14 +4,21 @@ on:
   push:
     branches: [main]
   pull_request:
+  workflow_dispatch:
 
 permissions:
   contents: read
 
 jobs:
   verify:
-    name: Verify and package
+    name: Verify and package (${{ matrix.os }})
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
+    strategy:
+      # Report Linux and Windows results independently instead of cancelling
+      # the matrix on the first failure.
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, windows-latest]
     steps:
       - name: Checkout
         uses: actions/checkout@v5