Add 0x03_button_cbm: bare-metal RP2350 button driver

- GPIO15 active-low button input with internal pull-up
- 20ms software debounce via busy-wait confirmation
- LED mirrors button state, UART reports edge transitions
- New gpio_config_input_pullup() in GPIO driver
- 1555B FLASH, 13 source files, zero warnings

drivers/0x03_button_cbm/Inc/rp2350_stack.h

@@ -0,0 +1,34 @@
+/**
+  ******************************************************************************
+  * @file    rp2350_stack.h
+  * @author  Kevin Thomas
+  * @brief   Stack pointer initialization header for RP2350.
+  *
+  *          Sets MSP, PSP, MSPLIM, and PSPLIM from the STACK_TOP and
+  *          STACK_LIMIT values defined in rp2350.h.
+  *
+  ******************************************************************************
+  * @attention
+  *
+  * Copyright (c) 2026 Kevin Thomas.
+  * All rights reserved.
+  *
+  * This software is licensed under terms that can be found in the LICENSE file
+  * in the root directory of this software component.
+  * If no LICENSE file comes with this software, it is provided AS-IS.
+  *
+  ******************************************************************************
+  */
+
+#ifndef __RP2350_STACK_H
+#define __RP2350_STACK_H
+
+#include "rp2350.h"
+
+/**
+  * @brief  Initialize MSP, PSP, MSPLIM, and PSPLIM stack pointers.
+  * @retval None
+  */
+void stack_init(void);
+
+#endif /* __RP2350_STACK_H */