Linux64 / SteamOS native window test (#459)

This complete test is based on v2.3-test515 and adds PR520's native64 host diagnostics and corrected SDL X11 runtime names. The game core still identifies as v2.3-test515; this test changes the64-bit host/runtime, not gameplay or imported media. Stable v2.2.5 and earlier downloads remain unchanged.

1. Extract the complete Linux archive into a new folder and put your own ROM in roms/.
2. Point your usual native Steam shortcut at this folder's ReKHCoM.sh; keep forced Proton disabled.
3. Remove --safe-startup / --force32 for this check and use Automatic. Try launching normally.
4. Tell us whether it starts, how long it takes, and whether HD text still works. Attach this new folder's ReKHCoM.log and kh_com.log if possible.

The confirmed compatible32/software recovery is retained. If the new normal launch still fails, --safe-startup remains available. Keep your working installation and saves; no new HD-media import is needed for this startup test.

Local validation found two Xlib objects with the previous SDL when system Xlib was preloaded, and one after correction. The new host entered the real64-bit game/title using X11/OpenGL with and without preloading in a snapshot VM. This is a tentative fix for the reporter's window hang, not physical SteamOS confirmation. The pre-guest30-second guard remains in place; we have not merely reduced its delay or disabled GPU.