feat(boards): Add secondary button support to Waveshare ESP32-C6-Touch-AMOLED-2.16

- Enable BTN_FWD_GPIO (GPIO 10) as secondary button for mode toggle (Shift+Tab)
- Update BOARD_HAS_SECONDARY_BUTTON flag from 0 to 1
- Clarify button layout: BOOT (primary PTT), PWR (cycle screens via AXP), KEY (secondary mode toggle)
- Document KEY GPIO discovery method (empirical scanning of unused GPIOs)
- Update button_count calculation to include secondary button when available
- Initialize both BTN_BACK_GPIO and BTN_FWD_GPIO in input_hal_init()
- Refactor input_hal_is_held() to use switch statement for cleaner multi-button handling

firmware/src/boards/waveshare_amoled_216_c6/board.h

@@ -42,12 +42,17 @@
 #define AXP2101_ADDR         0x34
 
 // ---- Buttons ----
-// Only one user GPIO button (BOOT). The "PWR" side button is the AXP2101
-// PKEY input — already serviced by power.cpp via the PKEY_SHORT_IRQ path.
-#define BTN_BACK_GPIO        9     // BOOT — primary, Space (PTT)
+// Three side-mounted buttons:
+//   BOOT (primary) — GPIO 9, sends Space (PTT) over BLE HID
+//   PWR  (cycle screens) — AXP2101 PKEY IRQ, handled in power.cpp
+//   KEY  (secondary) — GPIO 10, sends Shift+Tab (mode toggle) over BLE HID
+// KEY GPIO isn't documented by Waveshare; identified empirically by
+// scanning unused GPIOs at boot (see git history of board_init.cpp).
+#define BTN_BACK_GPIO        9
+#define BTN_FWD_GPIO         10
 
 // ---- Capability flags ----
-#define BOARD_HAS_SECONDARY_BUTTON 0
+#define BOARD_HAS_SECONDARY_BUTTON 1
 #define BOARD_HAS_ROTATION         0    // C6 has no PSRAM headroom for the rotation strip
 #define BOARD_HAS_IMU              1    // present + initialized for I2C bus health
 #define BOARD_HAS_BATTERY          1

firmware/src/boards/waveshare_amoled_216_c6/caps.cpp

@@ -5,7 +5,9 @@ static const BoardCaps caps = {
     .name = BOARD_NAME,
     .width = LCD_WIDTH,
     .height = LCD_HEIGHT,
-    .button_count = 1,    // BOOT only; PWR is via AXP PKEY IRQ
+    // BOOT (primary) + KEY (secondary) GPIO buttons. PWR is on the AXP
+    // PKEY and handled separately (not counted here).
+    .button_count = (uint8_t)(1 + BOARD_HAS_SECONDARY_BUTTON),
     .has_rotation = (bool)BOARD_HAS_ROTATION,
     .has_battery = (bool)BOARD_HAS_BATTERY,
     .has_imu = (bool)BOARD_HAS_IMU,

firmware/src/boards/waveshare_amoled_216_c6/input.cpp

@@ -2,16 +2,17 @@
 #include "board.h"
 #include <Arduino.h>
 
-// Only BOOT is wired to a MCU GPIO. The "PWR" side button is the AXP2101
-// PKEY, handled in power.cpp.
-
 void input_hal_init(void) {
     pinMode(BTN_BACK_GPIO, INPUT_PULLUP);
+    pinMode(BTN_FWD_GPIO,  INPUT_PULLUP);
 }
 
 bool input_hal_is_held(InputButton btn) {
-    if (btn == INPUT_BTN_PRIMARY) {
+    switch (btn) {
+    case INPUT_BTN_PRIMARY:
         return digitalRead(BTN_BACK_GPIO) == LOW;
+    case INPUT_BTN_SECONDARY:
+        return digitalRead(BTN_FWD_GPIO) == LOW;
     }
     return false;
 }