Issue #26: runtime master volume (-/= keys, persisted)

The game audio plays hot with no in-game control -- testers couldn't
hear voice chat without the Windows mixer (playtest night).  The -/=
keys now step the OpenAL listener gain (the master scale every source
inherits) in 5% steps, clamped 0..150%, edge-detected in the per-frame
poll with a foreground guard; the value persists to content\volume.cfg
and reloads at boot (BT_AUDIO_VOLUME env still wins when set; default
stays 0.6).  README updated.

Verified: volume.cfg 0.25 -> boot log "master gain=0.25"; key stepping
needs a live focused session (awaiting live verification).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

players/README.txt

@@ -41,6 +41,7 @@ CONTROLS (keyboard):
   1 or Space ........... main trigger          2 ............. middle thumb
   3 or Ctrl ............ upper thumb (missiles) 4 ............ pinky
   V view | B look-behind | M control mode | N schematic | J/K/L MFD pages
+  - / = ................ game volume down/up (saved between sessions)
   G (hold) + fire key .. regroup a weapon onto that trigger -- or hold a
      weapon panel's red PROGRAM button with the mouse and tap a fire key;
      the joystick diagram on the panel shows the binding live.