The previous override targeted gamescope-session-plus only. On stock
SteamOS (/usr/lib/steamos/gamescope-session) that override is never
sourced and the session forces MANGOHUD_CONFIGFILE=<tmp>/no_display,
so no logs were produced. Detect the session flavor and, on SteamOS,
set MANGOHUD_CONFIG in environment.d — it overrides the forced config
file and enables logging globally with no per-game launch options.
Verified with a vkcube precedence test (412-row CSV) and end-to-end in
a live SteamOS 3.9 Game Mode session (DREDGE, 1039-row mangoapp CSV).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
