Add 0x04_pwm_cbm: bare-metal RP2350 PWM driver

- PWM slice 4, channel B on GPIO25 (onboard LED)
- Duty cycle sweep 0-100% in 5% steps for breathing effect
- Wrap 9999 with no clock division (~650 Hz from ROSC)
- UART reports each duty step
- 1390B FLASH, 11 source files, zero warnings

drivers/0x04_pwm_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 */