refactor(boards): Move Proton-C to ZMK variant by default

Make the standard Proton-C board ID be `proton_c/stm32f303xc/zmk` or
`proton_c//zmk` by shorthand.

app/boards/qmk/proton_c/board.yml

@@ -1,5 +1,5 @@
 board:
-  name: proton_c
-  vendor: qmk
-  socs:
-    - name: stm32f303xc
+  extend: proton_c
+  variants:
+    - name: zmk
+      qualifier: stm32f303xc

app/boards/qmk/proton_c/proton_c.zmk.yml

@@ -1,5 +1,5 @@
 file_format: "1"
-id: proton_c
+id: proton_c//zmk
 name: QMK Proton-C
 type: board
 arch: arm

app/boards/qmk/proton_c/proton_c_stm32f303xc_zmk.dts

@@ -0,0 +1,9 @@
+/*
+ * Copyright (c) 2026 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+/dts-v1/;
+#include <../boards/qmk/proton_c/proton_c.dts>
+

app/core-coverage.yml

@@ -1,7 +1,7 @@
 board:
   - nice_nano
   - nrfmicro/nrf52840/zmk
-  - proton_c
+  - proton_c//zmk
 shield:
   - corne_left
   - corne_right
@@ -16,7 +16,7 @@ include:
   - board: nrf52840_m2//zmk
     shield: m60
   - board: planck//zmk
-  - board: proton_c
+  - board: proton_c//zmk
     shield: clueboard_california
   - board: nice_nano
     shield: kyria_left

app/module/boards/qmk/proton_c/board.cmake

@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: MIT
+
+board_runner_args(dfu-util "--pid=0483:df11" "--alt=0" "--dfuse")
+board_runner_args(jlink "--device=STM32F303CC" "--speed=4000")
+
+include(${ZEPHYR_BASE}/boards/common/dfu-util.board.cmake)
+include(${ZEPHYR_BASE}/boards/common/jlink.board.cmake)

app/module/boards/qmk/proton_c/board.yml

@@ -0,0 +1,5 @@
+board:
+  name: proton_c
+  vendor: qmk
+  socs:
+    - name: stm32f303xc

app/module/boards/qmk/proton_c/proton_c.zmk.yml

@@ -0,0 +1,9 @@
+file_format: "1"
+id: proton_c
+name: QMK Proton-C
+type: board
+arch: arm
+outputs:
+  - usb
+url: https://qmk.fm/proton-c/
+exposes: [pro_micro]

app/module/boards/qmk/proton_c/proton_c_defconfig

@@ -0,0 +1,16 @@
+# SPDX-License-Identifier: MIT
+
+# 72MHz system clock
+CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC=72000000
+
+# Floating Point Options
+CONFIG_FPU=y
+
+# enable pinctrl
+CONFIG_PINCTRL=y
+
+# enable GPIO
+CONFIG_GPIO=y
+
+# clock configuration
+CONFIG_CLOCK_CONTROL=y