Eject firsthand prep: G = coolant Flush binding + BT_FLUSH_HOLD scalpel

The Flush ACTION existed in the input engine but no key was bound in the
shipped CONTROLS.MAP -- desktop players could never hold the flush. G (free)
now maps to it. BT_FLUSH_HOLD=<frames> extends the BT_FLUSH_TEST scalpel
past its 60-frame edge test for tank-dry benches.

Dry-run finding (fl.log): a full flush drains the tank to 0/14 in ~30 s,
but does NOT arm eject by itself -- InjectCoolant moves tank coolant INTO
the loops (the bank RISES), and idle heat draw is too slow to burn the
loops down to the 5% arm threshold in minutes. Firsthand arming needs
sustained fire after tank-dry (or coolant-loop leak damage / generator
loss). Matches the binary's intent: eject arms when LEAKED nearly dry.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

content/CONTROLS.MAP

@@ -165,6 +165,12 @@ pad Start action ModeCycle
 pad RightThumb action DisplayCycle
 pad Back  action Valve
 
+# COOLANT FLUSH -- HELD key dumps reservoir coolant into the loops
+# (Reservoir InjectCoolant, msg 4).  The tank drains while held; with the
+# tank empty the loops burn coolant with no refill -- run hot long enough
+# and the bank drops below 5%, which ARMS the eject (Panic lights).
+key G action Flush
+
 # PANIC / EJECT -- the punch-out (Mech msg 0x19).  Only fires on a CRIPPLED
 # mech (no generators, coolant leaked below 5%, or leg-gimped novice) -- a
 # healthy mech refuses the button, so a stray press is a no-op.