Bootstrap Windows runtime files during local make builds

[dgruss]

Windows builds currently depend on generated runtime files that are not tracked in the repository. After src/config-win.inc moved out of the repository, local Windows builds fail unless developers manually pulled the DLL archive and extracted both the runtime DLLs and config include. This was already a problem with the DLLs before but they could be copied from any copy of the game or the release zip. The config-win.inc not, so the problem got worse and we should address this.
The solution is to make the local make path follow the same dependency bootstrap as CI. The CI already performs the bootstrap steps.

Changes

• add dldlls.py --ensure to download and extract the Windows DLL bundle plus src/config-win.inc when missing
• run the helper before Windows .exe builds from src/Makefile.in
• reuse the same helper in GitHub Actions instead of separate 7z extraction commands
• document the local Windows build behavior