Engine 11.9 bundled - verify before promoting to prod

[github-actions]

Engine 11.9 has been built, signed, smoke-tested, and uploaded as a GitHub Actions artifact.

Verify before promoting to prod:

• Fresh install: setup flow downloads and installs engine cleanly
• Create a test bottle and run a known-working .exe
• DXVK loads correctly
• Engine update flow: uninstall engine, relaunch, update offered and installs
• When verified, run the Promote engine to prod workflow (engine-promote-prod.yml)