STM32WB55 Support and Reliable Cross-Family DFU Implementation

## Description

Major update to STM32 support introducing STM32WB55 support and a complete overhaul of software DFU entry across supported STM32 families.

## Added
- STM32WB55 support (tested on WeAct STM32WB55CGU6)
- Support via P-NUCLEO-WB55 and NUCLEO WB55 USB Dongle board definitions
- Nucleo-64 board definition support (F1/F4/G4 variants should work)
- `dfu_boot_stm32wb.c` for reliable WB55 DFU entry
- Additional hardware validation: WeAct STM32G474 and STM32F405

## Changed
- DFU implementation rewritten for reliability:
  - F1/F4/G4 use direct ROM bootloader jump
  - WB55 uses magic value in `BKP0R` + `NVIC_SystemReset()` with early Reset_Handler interception
- `boards.txt` now requires extra lines to enable 1200bps touch
- Updated `boards_txt_additions.txt` with new format and Nucleo_64 entry
- Added suggested change to readme to reflect status of this port

## Known Limitations
- Generic WB55 board definitions lack proper USB clock configuration and are not supported. Use P-NUCLEO-WB55 or NUCLEO WB55 USB Dongle board definitions.
- STM32F1 DFU entry requires further validation on boards with factory bootloader.

This release significantly improves stability and expands supported hardware.

Author: code-fiasco

README.md

@@ -47,20 +47,18 @@ Note: For ESP32 port, version before v3.0 requires all descriptors must be speci
 
 ### Cores without built-in support
 
-Following is cores without built-in support
+The following are cores without built-in support
 
-- **mbed_rp2040**
 - **[stm32duino/Arduino_Core_STM32](https://github.com/stm32duino/Arduino_Core_STM32)**
-  - Still WIP, only supports F1, F4 & G4 so far. Requires lines added to stm32duino boards.txt which can be found in /ports/stm32/boards_additions.txt. No longer needs manual initialization/CDC flushing. DFU entry working. Serial now works as normal. 
-
+  - Still WIP. Fully functional but with limited number of MCUs supported so far (F1xx, F4xx, some G4xx & WB55). Requires lines added to stm32duino boards.txt which can be found in /ports/stm32/boards_txt_additions.txt. 
 
+- **mbed_rp2040**
+  - It is still possible to use TinyUSB but with some limits such as:
+    - `TinyUSB_Device_Init()` need to be manually called in setup()
+    - `TinyUSB_Device_Task()` and/or `TinyUSB_Device_FlushCDC()` may (or not) need to be manually called in loop()
+    - Use `SerialTinyUSB` name instead of Serial for serial monitor
 
-It is still possible to use TinyUSB but with some limits such as:
-
-- `TinyUSB_Device_Init()` need to be manually called in setup()
-- `TinyUSB_Device_Task()` and/or `TinyUSB_Device_FlushCDC()` may (or not) need to be manually called in loop()
-- Use `SerialTinyUSB` name instead of Serial for serial monitor
-- And there could be more other issues, using on these cores should be considered as experimental
+There could be more other issues on cores without built in support, using on these cores should be considered experimental.
 
 ## Class Driver API