fix: map throttle to axis 3 (AETR layout)

Verified via hid_discover caps and live data dump. Throttle
responds on axis 3 (HID usage 0x33 = Rx), not axis 5 or 6.

README.md

@@ -109,8 +109,9 @@ int16_t ry = stk_right_y(&state);
 int16_t throttle = stk_throttle(&state);
 int16_t lx = stk_left_x(&state);
 
-// Raw axes array (indices 0-7, only 1/2/4/5 active)
+// Raw axes array (indices 0-7, only 1/2/3/4 active)
 // state.axes[0..7] — values in range STK_AXIS_MIN..MAX (0..2047)
+// AETR layout: axes 1/2 = right stick, 3 = throttle, 4 = left X (rudder)
 
 // Button check
 if (stk_button_pressed(&state, 3)) {
@@ -127,10 +128,13 @@ EdgeTX radio (VID:PID 1209:4F54) HID input report:
 
 | Field   | Size       | Details                                             |
 | ------- | ---------- | --------------------------------------------------- |
-| Axes    | 4 x 16-bit | Range 0-2047 (HID usage 0x30-0x37, indices 1/2/4/5) |
+| Axes    | 4 x 16-bit | Range 0-2047 (HID usage 0x30-0x37, indices 1/2/3/4) |
 | Buttons | 24 bits    | Switches, rockers, paddles                          |
 | Total   | 20 bytes   | No report ID                                        |
 
+Axis 1 = right X (aileron), 2 = right Y (elevator), 3 = throttle,
+4 = left X (rudder). This matches the AETR stick layout.
+
 ## Current Status
 
 - [x] Build system (CMake + Ninja)