refactor: convert 0x01_uart_cbm and 0x02_blink_cbm to bare-metal Inc/Src structure

- Restructure flat files into Inc/ (headers) and Src/ (sources)
- Replace constants.h with rp2350.h register layer (datasheet-verified)
- Add full Doxygen docstrings on all files, functions, and structs
- Replace build.bat/clean.bat with cross-platform Makefile
- Fix GPIO_IN offset (0x004), XOSC COUNT offset (0x10), SRAM size (520K)
- Rename blink_* API to led_* (name after peripheral, not feature)
- Build outputs to build/ directory
- Cross-platform .vscode configs (Windows/macOS/Linux)

drivers/0x01_uart_cbm/Src/image_def.c

@@ -0,0 +1,35 @@
+/**
+  ******************************************************************************
+  * @file    image_def.c
+  * @author  Kevin Thomas
+  * @brief   RP2350 IMAGE_DEF block for boot ROM image recognition.
+  *
+  *          Must appear within the first 4 KB of flash for the boot ROM
+  *          to accept the image.
+  *
+  ******************************************************************************
+  * @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>
+
+/**
+  * @brief IMAGE_DEF block structure placed in flash
+  */
+__attribute__((section(".embedded_block"), used))
+const uint8_t picobin_block[] = {
+  0xD3, 0xDE, 0xFF, 0xFF,
+  0x42, 0x01, 0x21, 0x10,
+  0xFF, 0x01, 0x00, 0x00,
+  0x00, 0x00, 0x00, 0x00,
+  0x79, 0x35, 0x12, 0xAB
+};