README: what the game needs from SteamOS

Checked on a Frame with ldd: the executable needs only glibc, libstdc++,
libgcc, libpng and zlib from the system, and no symbol version newer than
SteamOS 0.4.3's glibc 2.39 offers, although the build container has 2.41.
So it runs both from Steam, inside Steam Linux Runtime 4, and straight
from a Desktop Mode terminal.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019HBRGKTE1GnN2ah8gcZKr3

README.md

@@ -375,6 +375,12 @@ Unraid keeps `/usr/local/bin` in memory, so repeat this after a reboot.
   Vulkan hook and density map ABIs that Aurora compiles against.
 - **Headset only.** The game neither shows nor finishes rendering a desktop window, and is compiled
   for the Frame's Cortex-X4 cores (`-mcpu=cortex-x4`, overridable with `--cpu`).
+- **What it needs from SteamOS.** Dawn, SDL and the OpenXR loader are linked in, and Vulkan and the
+  OpenXR runtime are found at run time. From the system it takes only glibc, libstdc++, libgcc,
+  libpng and zlib, all part of SteamOS, and it is built against glibc symbols that SteamOS 0.4.3's
+  glibc 2.39 has (`ldd ~/devkit-game/WiiCompiled/WiiCompiled` lists them and flags anything
+  missing). Steam starts it in Steam Linux Runtime 4 (`SteamLinuxRuntime_4-arm64`), which brings
+  its own copies; started from a terminal it uses SteamOS's.
 - **Every refresh from the game.** The game draws 60 frames a second. When it handed SteamVR only
   those, SteamVR ran it at half rate and made up every other refresh itself, even with Motion
   Smoothing off. With `[vr] repeat_frames`, the pacing thread waits for the next frame until 1.5 ms