SDL Adds DOS Support

This is retro-computing with real engineering weight, not a gimmick.

• –The port is broad enough to matter: it covers the core runtime subsystems most ports fail on, not just a minimal build.
• –DJGPP support plus a CMake toolchain and CI job make it much easier for emulator and retro-game developers to target DOS seriously.
• –The PR is candid about limits and testing; no real-hardware validation means this is impressive, but not fully battle-tested yet.
• –For SDL, this reinforces its role as a portability layer that keeps expanding to unusual platforms instead of narrowing to modern ones.