Issue #25: composite Turn channel -- twin-stick gamepad turning

Gamepad players expect stick X = turn; the pod-faithful default maps it
to torso twist, and the pedal pair couldn't take a single signed axis.
New bindings channel "Turn": a signed composite that decomposes into
the pedal pair at the RIO publish (+ = right pedal, - = left), ADDING
to direct pedal bindings so mixed rigs compose, clamped 0..1 per pedal.
The default bindings.txt stays pod-faithful and documents the
twin-stick alternative inline:

    padaxis LX axis Turn
    padaxis RX axis JoystickX

Also covers the HOTAS ask (any signed axis can now drive turning).
Verified: regenerated defaults carry the doc lines; a twin-stick-edited
bindings.txt parses and boots clean.  Awaiting live pad verification.

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

engine/MUNGA_L4/L4PADBINDINGS.cpp

@@ -98,6 +98,7 @@ static const NamedValue channelNames[] =
 	{ "JoystickY",	PadBindingProfile::ChannelJoystickY },
 	{ "LeftPedal",	PadBindingProfile::ChannelLeftPedal },
 	{ "RightPedal",	PadBindingProfile::ChannelRightPedal },
+	{ "Turn",		PadBindingProfile::ChannelTurn },		// issue #25 composite
 };
 
 static int
@@ -166,7 +167,9 @@ static const char *defaultProfileText =
 "#   key     <KEYNAME> axis <channel> set <value>            jump/detent\n"
 "#   pad     <PADBTN>  button <addr>\n"
 "#   padaxis <PADAXIS> axis <channel> [invert] [slew <rate>]\n"
-"# Channels: Throttle JoystickX JoystickY LeftPedal RightPedal\n"
+"# Channels: Throttle JoystickX JoystickY LeftPedal RightPedal Turn\n"
+"#   (Turn = signed composite: + drives the right pedal, - the left --\n"
+"#    made for mapping a gamepad stick to turning)\n"
 "# Pad buttons: A B X Y LB RB BACK START LS RS DPAD_*  Axes: LX LY RX RY LT RT\n"
 "#\n"
 "# LAYOUT: the keyboard hosts the core gameplay actions on the desktop\n"
@@ -231,6 +234,10 @@ static const char *defaultProfileText =
 "key F9 button 0x22\n"
 "\n"
 "# --- XInput pad ---\n"
+"# POD-FAITHFUL (default): stick X = torso twist, triggers = pedals.\n"
+"# TWIN-STICK alternative: replace the LX line with the two below --\n"
+"#   padaxis LX axis Turn\n"
+"#   padaxis RX axis JoystickX\n"
 "padaxis LX axis JoystickX\n"
 "padaxis LY axis JoystickY invert\n"
 "padaxis LT axis LeftPedal\n"