ci: Migrate the Windows x64 job to Visual Studio 2026 on "windows-2025"

The "windows-latest" alias was repointed to a Windows Server 2025 image
that ships Visual Studio 2026 only, which prevented the version-exact
generator "Visual Studio 17 2022" from matching any installed instance.

This is a cherry-pick of commit dd5d363ae1fc7778f2734bf51b10d3fe65028671
from branch 'libpng18'.

.github/workflows/verify-windows.yml

diff --git a/.github/workflows/verify-windows.yml b/.github/workflows/verify-windows.yml
index 0bedb8a..5f9bd33 100644
--- a/.github/workflows/verify-windows.yml
+++ b/.github/workflows/verify-windows.yml
@@ -18,9 +18,9 @@
     strategy:
       matrix:
         include:
-          - os: windows-latest
+          - os: windows-2025
             vcpkg_triplet: x64-windows
-            cmake_generator: Visual Studio 17 2022
+            cmake_generator: Visual Studio 18 2026
             cmake_generator_platform: x64
           - os: windows-11-arm
             vcpkg_triplet: arm64-windows