ci: restore macOS and move all runners back to WarpBuild

CI matrix: warp-ubuntu-latest-x64-8x / warp-macos-15-arm64-6x /
warp-windows-latest-x64-16x (the macOS steps were still in place, only
the matrix entry was missing). CD: macOS DMG job re-enabled on the warp
Apple Silicon runner; linux and release jobs back on warp-ubuntu;
Windows NSIS stays disabled until the engine links there.

.github/workflows/ci.yml

@@ -23,8 +23,9 @@ jobs:
       fail-fast: false
       matrix:
         os:
-          - ubuntu-latest
-          - windows-latest
+          - warp-ubuntu-latest-x64-8x
+          - warp-macos-15-arm64-6x
+          - warp-windows-latest-x64-16x
 
     steps:
       - name: Checkout