HUD (task #68 follow-up): authentic reticle pip GROUPS -- front vs rear

The binary reticle registration resolves each weapon's RearFiring attribute
and registers its pip in group 1 (front) or 2 (rear) (part_014.c:5429-5434)
[T1]; the reticle draws the group selected by the mech's reticleElementMask
low bits (= binary mech+0x390, driven by the weapon-update view switch
part_013.c:5588-5595: forward |1&~2, look-back |2&~1).

Port: BTBuildReticle passes the real group (was hardcoded 1 on the disproven
"no BLH weapon is rear" belief); BTCommitLookState drives the mech's
reticleElementMask low bits per view (mech4's HUD tick already publishes
them as gBTHudGroupMask, and the reticle Draw already filters on it).

Resolves the user-reported "only 2 ERM dots but the mech has 3": ERMLaser_1
(front) and ERMLaser_2 (rear) both author pip position 1 -- with every pip
drawn in one group the two red dots sat exactly on top of each other.  Now:
forward view = 5 pips (2 PPC + ERM_1 + 2 SRM), look-back = the 2 rear
lasers' own group layout.

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

game/reconstructed/btl4vid.cpp

@@ -1902,7 +1902,12 @@ BTReticleRenderable *BTBuildReticle(Entity *mech)
 			2, 3,
 			(int *)wp->SimulationStatePtr(),		// attr 1: damage state (1 = destroyed)
 			1,
-			1 /* Front group (RearFiring==0 on every BLH weapon) */);
+			// (task #68) the AUTHENTIC pip group: the binary registration reads
+			// the weapon's RearFiring attr and passes group 1 (front) / 2 (rear)
+			// (part_014.c:5429-5434) -- the reticle draws the group matching
+			// the current look view (mech reticleElementMask low bits).  The
+			// old hardcoded 1 rode the disproven "no BLH weapon is rear" belief.
+			wp->GetRearFiring() ? 2 : 1);
 	}
 	DEBUG_STREAM << "[hud] reticle built: " << gBTReticle->WeaponCount()
 				 << " weapon pip(s) registered\n" << std::flush;

game/reconstructed/mech4.cpp

@@ -6225,6 +6225,17 @@ void BTCommitLookState(void *mech_v, int look_state)
 				(look_state == 3) ? rf : 0);
 		}
 	}
+
+	// (task #68) the HUD pip GROUP mask (binary mech+0x390, weapon update
+	// part_013.c:5588-5595): forward view shows the FRONT pip group (|=1,&~2),
+	// look-back shows the REAR group (|=2,&~1); side/down views leave it.
+	// mech4's HUD tick publishes the low bits as gBTHudGroupMask each frame.
+	if (look_state == 0)
+		m->targetReticle.reticleElementMask = (Reticle::ReticleElements)
+			(((int)m->targetReticle.reticleElementMask | 1) & ~2);
+	else if (look_state == 3)
+		m->targetReticle.reticleElementMask = (Reticle::ReticleElements)
+			(((int)m->targetReticle.reticleElementMask | 2) & ~1);
 	if (getenv("BT_KEY_LOG"))
 		DEBUG_STREAM << "[look] state=" << look_state
 			<< " rearFiring=" << m->rearFiring