Issue #20: wire Mech BalanceCoolant (id 0x16) -- the auto-cooling-balance button

Third instance of the silent-swallow pattern (unregistered handler id ->
Receiver NullHandler).  The Mech handler table @0x50BDF8 binds
{0x16, "BalanceCoolant" -> @0049f728}; the reconstruction never registered it.

Disasm-verified body (@0049f728): press-only guard (msg+0xc > 0; the binary
has NO novice lockout on this one), set EVERY condenser's valveState@0x1D0
to 1 (equal weights), then the shared redistribute @0049f788 -- which the
port ALREADY had faithfully as BTRecomputeCondenserValves (flow =
valve/total + condenserAlarm pulse; MoveValve calls it).  So the fix is:
- mech.hpp/mech.cpp: BalanceCoolantMessageID=0x16 + handler + MESSAGE_ENTRY
- heatfamily_reslice.cpp: BTBalanceCondenserValves bridge (Condenser is a
  complete type there; sets valves to 1 + redistributes)
- mech4.cpp: BT_BALTEST scripted harness (MoveValve press alternating with
  a Balance press)

VERIFIED headless (BT_BALTEST=1 BT_VALVE_LOG=1):
  boot     -> all valves 1, flows 0.1667 each
  MoveValve-> condenser#1 valve 5, flows 0.5 / 0.1x5 (the priority boost)
  BALANCE  -> all valves 1, flows 0.1667 each (equalized)
Awaiting the tester's live ADV-mode confirmation.

KB: decomp-reference.md Mech-table row marked wired.

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

game/reconstructed/mech4.cpp

@@ -5654,6 +5654,43 @@ void
 			}
 		}
 
+		// issue #20 verify (BT_BALTEST): alternate a MoveValve (id 4) press to
+		// the first condenser with a Mech BalanceCoolant (id 0x16) press every
+		// ~2s -- BT_VALVE_LOG shows the valve going 1->5 then BALANCE pulling
+		// every valve back to 1 with equal flows.
+		if (getenv("BT_BALTEST") && (Entity *)this == application->GetViewpointEntity())
+		{
+			static int s_balTest = 0;
+			if ((++s_balTest % 120) == 0)
+			{
+				int phase = (s_balTest / 120) & 1;
+				if (phase == 0)
+				{
+					for (int s = 1; s < GetSubsystemCount(); ++s)
+					{
+						Subsystem *sub = GetSubsystem(s);
+						if (sub != 0 && (int)sub->GetClassID() == 0xBBD)	// Condenser
+						{
+							ReceiverDataMessageOf<ControlsButton> msg(
+								4 /*Condenser::MoveValveMessageID*/,
+								sizeof(ReceiverDataMessageOf<ControlsButton>),
+								(ControlsButton)1);
+							sub->Dispatch(&msg);
+							break;
+						}
+					}
+				}
+				else
+				{
+					ReceiverDataMessageOf<ControlsButton> msg(
+						0x16 /*Mech::BalanceCoolantMessageID*/,
+						sizeof(ReceiverDataMessageOf<ControlsButton>),
+						(ControlsButton)1);
+					Dispatch(&msg);
+				}
+			}
+		}
+
 		// task #6 dev harness: the config-session BRACKET.  'G' edge ->
 		// dispatch ConfigureMappables (id 9) press/release to the selected
 		// weapon -- the exact ReceiverDataMessageOf<ControlsButton> payload