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)

stk/stk.h

@@ -14,16 +14,15 @@
  *   bytes 18..19 : buttons bits 8..23
  * @endcode
  *
- * @par Active axes:
+ * @par Active axes (AETR stick mapping):
- * | Index | Usage | Physical control | Rest value |
+ * | Index | Usage | HID name | Physical control | Rest value |
- * |-------|-------|-----------------|------------|
+ * |-------|-------|----------|-----------------|------------|
- * | 0     | —     | unused          | always 0   |
+ * | 0     | 0x30  | X        | unused          | always 0   |
- * | 1     | RX    | right stick X   | ~1024      |
+ * | 1     | 0x31  | Y        | right stick X   | ~1024      |
- * | 2     | RY    | right stick Y   | ~1024      |
+ * | 2     | 0x32  | Z        | right stick Y   | ~1024      |
- * | 3     | —     | unused          | always 0   |
+ * | 3     | 0x33  | Rx       | throttle        | ~0         |
- * | 4     | Z     | left stick Y    | 0..2047    |
+ * | 4     | 0x34  | Ry       | left stick X    | ~1024      |
- * | 5     | VR    | left stick X    | ~1024      |
+ * | 5..7  | —     | unused   | always 0        | always 0   |
- * | 6..7  | —     | unused          | always 0   |
  */
 #pragma once
 
@@ -56,8 +55,8 @@ typedef struct {
 // ---------------------------------------------------------------------------
 // Radio state
 //
-// axes[] holds all 8 HID axes (indices 0-7). Only indices 1, 2, 4, 5
+// axes[] holds all 8 HID axes (indices 0-7). Only indices 1, 2, 3, 4
-// are wired to physical controls. The rest stay at 0.
+// are wired to physical controls (AETR layout). The rest stay at 0.
 //
 // buttons is a 24-bit bitmask: bit N set = switch/button N+1 pressed.
 // ---------------------------------------------------------------------------
@@ -83,11 +82,11 @@ static inline int16_t stk_right_y(stk_state_t const* s) {
 }
 
 static inline int16_t stk_throttle(stk_state_t const* s) {
-    return s->axes[4];
+    return s->axes[3];
 }
 
 static inline int16_t stk_left_x(stk_state_t const* s) {
-    return s->axes[5];
+    return s->axes[4];
 }
 
 // ---------------------------------------------------------------------------

tests/test_stk.c

@@ -75,8 +75,8 @@ void test_named_accessors_map_correct_indices(void) {
     stk_state_t state = { 0 };
     state.axes[1] = 111;
     state.axes[2] = 222;
-    state.axes[4] = 333;
+    state.axes[3] = 333;
-    state.axes[5] = 444;
+    state.axes[4] = 444;
 
     TEST_ASSERT_EQUAL_INT16(111, stk_right_x(&state));
     TEST_ASSERT_EQUAL_INT16(222, stk_right_y(&state));