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/Src/rp2350_stack.c

@@ -0,0 +1,38 @@
+/**
+  ******************************************************************************
+  * @file    rp2350_stack.c
+  * @author  Kevin Thomas
+  * @brief   Stack pointer initialization for RP2350.
+  *
+  *          Sets MSP, PSP, MSPLIM, and PSPLIM using inline assembly.
+  *
+  ******************************************************************************
+  * @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.
+  *
+  ******************************************************************************
+  */
+
+#include "rp2350_stack.h"
+
+void stack_init(void)
+{
+  __asm__ volatile (
+    "ldr   r0, =%0\n\t"
+    "msr   PSP, r0\n\t"
+    "ldr   r0, =%1\n\t"
+    "msr   MSPLIM, r0\n\t"
+    "msr   PSPLIM, r0\n\t"
+    "ldr   r0, =%0\n\t"
+    "msr   MSP, r0\n\t"
+    :
+    : "i" (STACK_TOP), "i" (STACK_LIMIT)
+    : "r0"
+  );
+}