feat: add SteamRT3 client incompatibility warning check

[hotline1337]

Detect unsupported SteamRT3 client and warn at install time

Closes #840

What this does

Adds a check_steamrt3_incompatibility function that runs after installing to warn the user when Steam is using the SteamRT3 (64-bit) experimental client, since the current preload hook targets the 32-bit client process (ubuntu12_32/libXtst.so.6) which RT3 never launches.

As described in the linked issue, without this check the install completes silently and the user has no way to tell "broken install" from "unsupported client".

Detection logic

Detection is done via three on-disk markers that together indicate an active RT3 session:

• $STEAM/package/beta - beta channel is active
• $STEAM/.steam-enable-steamrt64-client - RT3 experimental toggle is on
• $STEAM/steamrt64/steam - the 64-bit client binary is present

All three must exist for the warning to fire, which avoids false positives on a stable/32-bit install.

Where it's wired in

• .install (AUR/pacman hook) - post_install() calls the check.
• install.sh (Other distributions) - same check ported to the shell installer.
• Nix module - added to extraProfile alongside the existing symlink logic, so it runs in the same shell context where $HOME is already correctly resolved for the logged-in user.

Install still proceeds either way - this is a warning, not a hard block, per the discussion in the issue.

Sample output

  WARNING: SteamRT3 client detected

  Millennium does not support the experimental/beta SteamRT3 client.
  For Millennium to work correctly, please either:
  - disable the "Use experimental SteamRT3 Steam Client" option
  - opt out of the Steam Beta entirely

Testing

Tested on Arch with Steam publicbeta + RT3 experimental enabled (the environment from the original issue) - warning fires correctly post-install.