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/vector_table.c

@@ -0,0 +1,38 @@
+/**
+  ******************************************************************************
+  * @file    vector_table.c
+  * @author  Kevin Thomas
+  * @brief   Vector table with initial stack pointer and reset handler.
+  *
+  *          Placed in the .vectors section at the start of flash.
+  *          The Thumb bit (bit 0 = 1) is automatically set by the
+  *          linker for function pointers in Thumb mode.
+  *
+  ******************************************************************************
+  * @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 <stdint.h>
+
+extern uint32_t _stack_top;
+extern void Reset_Handler(void);
+
+typedef void (*vector_func_t)(void);
+
+/**
+  * @brief Vector table placed in .vectors section
+  */
+__attribute__((section(".vectors"), used))
+const void *_vectors[2] = {
+  &_stack_top,
+  Reset_Handler
+};