Jog: bind right back buttons (R1/R2) to A axis hold-to-jog

The default USB pendant config exposed A only via the right stick
X axis (ABS_RX). Most Onefinity-shipped pendants only have one
usable stick, so A was effectively unreachable.

Map BTN_TR (0x137, upper-right back) to A+ and BTN_TR2 (0x139,
lower-right back) to A- while held. Speed scaling matches the
sticks (1/128, 1/32, 1/4, 1x via X/A/B/Y).

R1 was the vertical-axis-lock toggle; horizontal lock on L1 is
preserved, vertical lock is dropped to free R1.

src/py/bbctrl/Jog.py

@@ -51,12 +51,18 @@ class Jog(inevent.JogHandler):
                 "dir":      [1, -1, -1, 1],
                 "arrows":   [ABS_HAT0X, ABS_HAT0Y],
                 "speed":    [0x133, 0x130, 0x131, 0x134],
-                "lock":     [0x136, 0x137],
+                "lock":     [0x136],          # L1 = horiz-lock; R1/R2 now A axis
+                # Right back triggers drive the A axis while held:
+                #   BTN_TR  (0x137, upper-right) -> A+
+                #   BTN_TR2 (0x139, lower-right) -> A-
+                "a_pos":    0x137,
+                "a_neg":    0x139,
             }
         }
 
         super().__init__(config)
 
+        self.a_button = 0  # -1, 0, +1 from R1/R2 hold state
         self.v = [0.0] * 4
         self.lastV = self.v
         self.callback()
@@ -64,6 +70,27 @@ class Jog(inevent.JogHandler):
         self.processor = inevent.InEvent(ctrl.ioloop, self, types = ['js'])
 
 
+    def event(self, event, state, dev_name):
+        cfg = self.get_config(dev_name)
+        a_changed = False
+
+        if event.type == EV_KEY:
+            old = self.a_button
+            if event.code == cfg.get('a_pos'):
+                self.a_button = 1 if event.value else (
+                    0 if self.a_button == 1 else self.a_button)
+            elif event.code == cfg.get('a_neg'):
+                self.a_button = -1 if event.value else (
+                    0 if self.a_button == -1 else self.a_button)
+            if self.a_button != old: a_changed = True
+
+        super().event(event, state, dev_name)
+
+        # JogHandler.event() only fires changed() when stick axes move.
+        # Force a recompute when R1/R2 hold state flips.
+        if a_changed: self.changed()
+
+
     def up(self): self.ctrl.lcd.page_up()
     def down(self): self.ctrl.lcd.page_down()
     def left(self): self.ctrl.lcd.page_left()
@@ -91,3 +118,7 @@ class Jog(inevent.JogHandler):
         if self.speed == 3: scale = 1.0 / 4.0
 
         self.v = [x * scale for x in self.axes]
+
+        # R1/R2 buttons override the A axis (axes[3]) while held.
+        if self.a_button:
+            self.v[3] = self.a_button * scale