CI: use windows-latest runner instead of windows-2019

windows-2019 runners appear unavailable on forks; windows-latest
uses the current recommended image and has better availability.

Co-developed-by: Claude Code v2.1.90 (claude-sonnet-4-6)

Author: robtaylor

.github/workflows/build-windows.yml

@@ -8,7 +8,7 @@ jobs:
 
   build-windows:
 
-    runs-on: windows-2019
+    runs-on: windows-latest
 
     steps: