document BT_RADAR_FLASH_RED in the generated environ.ini

The red-flash knob shipped documented in the KB and the source comment but not
in the player-facing settings file -- an inconsistency with its sibling
BT_RIO_PROTRUDE, which does carry a block there.  It is a persistent player
preference, so environ.ini is exactly where it belongs.

Placed in the cockpit section after BT_COCKPIT_TINT, written as
'#BT_RADAR_FLASH_RED=0' -- commented, showing the OFF value, since the feature
is on by default and turning it off is the action a player would take.

NB BT_LAMPTEST is deliberately NOT added: environ.ini's own contract excludes
test/debug hooks ("they would silently persist across every session").  It
stays documented in context/glass-cockpit.md and at its source.

Verified: the block reads back out of the freshly built btl4.exe verbatim.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

game/btl4main.cpp

@@ -394,6 +394,14 @@ static const char *kEnvironIniDefault =
 "# Phosphor colour of the mono MFDs, RRGGBB.  Default is a green tube.\n"
 "#BT_COCKPIT_TINT=21FF42\n"
 "\n"
+"# A FLASHING button on the radar turns RED instead of yellow.  The radar's\n"
+"# buttons are all yellow, so a lamp calling for attention used to change only\n"
+"# in BRIGHTNESS -- easy to miss with eleven other yellow buttons around it.\n"
+"# Now the bright half of its flash is red, so an alert changes COLOUR.  Only\n"
+"# a flashing button is affected: a radar button doing nothing looks exactly as\n"
+"# it always did.  1 = on (the default), 0 = keep the yellow.\n"
+"#BT_RADAR_FLASH_RED=0\n"
+"\n"
 "# ---- Display ----------------------------------------------------------------\n"
 "\n"
 "# The cockpit is fitted into the window at ONE uniform scale, centred, with\n"