Smoother arms, and swap head tilt/turn ports

- ARM_STEP_DEG 8 -> 3 and a dedicated ARM_MIN_GAP_MS of 60, so the arm
  tracks the trigger instead of jumping between targets. Speed and spans
  left as they were.
- hub1 port B is the tilt motor, C is the turn motor - swapped from what
  the first build assumed.
- Zero the arm encoders once per boot rather than on every reconnect. The
  hub keeps its preset, and re-zeroing mid-session would redefine zero at
  whatever position the arms were in when the link dropped.

receiver/receiver.ino

@@ -26,10 +26,10 @@
  * ARMS ARE POSITION CONTROLLED:
  *   The triggers set an ANGLE, not a power level. Trigger released holds the
  *   arm at 0, fully depressed holds it at ARM_SPAN_*. The encoders are zeroed
- *   on connect, so BOTH ARMS MUST BE DOWN when the hub connects.
- *   Measured travel was about 275 motor degrees each way; the spans below are
- *   set short of that deliberately, so backlash cannot drive the motor into
- *   its end stop while holding.
+ *   once, on the first hub1 connect after boot, so BOTH ARMS MUST BE DOWN at
+ *   that moment. They are deliberately NOT re-zeroed on a reconnect - the hub
+ *   keeps its encoder preset, and re-zeroing mid-session would redefine zero
+ *   at whatever position the arms happened to be in.
  *
  * FILE ORDER MATTERS:
  *   The Arduino IDE injects generated function prototypes immediately before
@@ -48,8 +48,8 @@
 //
 //   hub 0 - lower body        hub 1 - upper body
 //     A  right track            A  left arm
-//     B  left track             B  head turn
-//     C  (free)                 C  head tilt
+//     B  left track             B  head tilt
+//     C  (free)                 C  head turn
 //     D  body lift              D  right arm
 static const char *HUB0_ADDR = "90:84:2b:61:f2:d7";
 static const char *HUB1_ADDR = "90:84:2b:61:e6:8c";
@@ -80,14 +80,20 @@ static const int DIR_HEAD_TURN   =  1;
 
 // Arm travel in MOTOR degrees, measured with tools/arm_calibrate.
 // Raw measurements were roughly +280 (left) and -275 (right). These are set
-// ~10% short so backlash cannot stall the motor against the top stop.
+// slightly short so backlash cannot stall the motor against the top stop.
 // The sign carries the direction - there is no DIR_ constant for the arms.
 static const int32_t ARM_SPAN_LEFT  =  250;
 static const int32_t ARM_SPAN_RIGHT = -250;
 
 static const int  ARM_SPEED     = 60;   // how fast it travels to the target
 static const byte ARM_MAX_POWER = 40;   // torque cap - keeps a jam survivable
-static const int  ARM_STEP_DEG  = 8;    // ignore target changes smaller than this
+static const int  ARM_STEP_DEG  = 3;    // ignore target changes smaller than this
+
+// Arms get their own command interval. They are position controlled, so the
+// target moves continuously with the trigger and needs updating more often
+// than a velocity axis does - at the per-port 100ms the arm sprints to each
+// target then sits idle, which feels like stepping.
+static const unsigned long ARM_MIN_GAP_MS = 60;
 
 // Per-axis power caps for the velocity-controlled axes.
 static const int TRACK_MAX = 100;   // LEGO speed range is -100..100
@@ -235,7 +241,7 @@ static void driveArm(HubLink &hl, byte port, int32_t target, ArmState &st) {
 
     unsigned long now = millis();
     if (labs((long)target - (long)st.lastTarget) < ARM_STEP_DEG) return;
-    if ((now - st.lastSentAt) < MOTOR_MIN_GAP_MS) return;
+    if ((now - st.lastSentAt) < ARM_MIN_GAP_MS) return;
     if ((now - hl.lastCmdAt) < HUB_MIN_GAP_MS) return;
 
 #if DEBUG_MOTORS
@@ -297,8 +303,10 @@ static void serviceHub(HubLink &hl) {
     }
 }
 
-// Define "arms down" as angle zero. Has to happen after hub1 connects, and
-// the arms must physically be at the bottom of their travel.
+// Define "arms down" as angle zero. Runs once per boot, after hub1 connects,
+// with the arms physically at the bottom of their travel. Not repeated on a
+// reconnect: the hub keeps its encoder preset, and re-zeroing mid-session
+// would redefine zero wherever the arms happened to be sitting.
 static void zeroArms() {
     if (gArmsZeroed || !gHubs[1].hub.isConnected()) return;
 
@@ -370,7 +378,7 @@ static void applyFrame(const Frame &f) {
     int rightTrack = stickToSpeed(-f.ry, TRACK_MAX) * gTrackSpeed / 100
                      * DIR_RIGHT_TRACK;
 
-    // L1 / R1 swing the head while held.
+    // L1 / R1 turn the head while held.
     int headTurn = ((f.buttons & BTN_R1) ? HEAD_MAX
                  :  (f.buttons & BTN_L1) ? -HEAD_MAX : 0) * DIR_HEAD_TURN;
 
@@ -388,8 +396,11 @@ static void applyFrame(const Frame &f) {
     driveMotor(gHubs[0], PORT_A, rightTrack, gPort[0]);
     driveMotor(gHubs[0], PORT_B, leftTrack,  gPort[1]);
     driveMotor(gHubs[0], PORT_D, bodyLift,   gPort[2]);
-    driveMotor(gHubs[1], PORT_B, headTurn,   gPort[3]);
-    driveMotor(gHubs[1], PORT_C, headTilt,   gPort[4]);
+
+    // hub1 B is the tilt motor and C is the turn motor - the reverse of what
+    // the first build assumed.
+    driveMotor(gHubs[1], PORT_B, headTilt,   gPort[3]);
+    driveMotor(gHubs[1], PORT_C, headTurn,   gPort[4]);
 
     driveArm(gHubs[1], PORT_A, leftTarget,  gArmLeft);
     driveArm(gHubs[1], PORT_D, rightTarget, gArmRight);
@@ -418,11 +429,8 @@ void loop() {
     serviceHub(gHubs[0]);
     if (gHubs[0].hub.isConnected()) serviceHub(gHubs[1]);
 
-    if (gHubs[1].hub.isConnected()) {
-        zeroArms();
-    } else {
-        gArmsZeroed = false;   // re-zero after a reconnect
-    }
+    // Zeroes once per boot. Deliberately not reset when the hub drops.
+    if (gHubs[1].hub.isConnected()) zeroArms();
 
     bool ready = gHubs[0].hub.isConnected() && gHubs[1].hub.isConnected();
     digitalWrite(STATUS_LED_PIN, ready ? HIGH : LOW);