Add XIAO Wio-SX1262 overlay; support SX12xx bandwidths (including 62.5 kHz) as default

- boards/seeed/xiao_wio_sx1262: add overlay, .conf, board.yml
- app: support full set of SX12xx bandwidths; default BW set to 62 (62.5 kHz)
- Update Kconfig and prj.conf
- Update lora_modem.h/c for validation and mapping

README.md

@@ -0,0 +1,67 @@
+# LoRa KISS Modem
+
+This project implements a KISS (Keep It Simple Stupid) modem for LoRa radios, targeting widely available commercial development boards (e.g., Heltec, RAK, Seeed, ESP32, nRF52840). It bridges a KISS serial interface to a LoRa radio, enabling packet radio applications over long-range wireless links.
+
+## Features
+- KISS TNC protocol over UART/USB serial
+- LoRa radio support (Semtech LR11xx and compatible)
+- Zephyr RTOS-based for portability and reliability
+- No dynamic memory allocation (static stack/heap usage)
+- Designed for low-power, embedded use
+
+## Hardware Requirements
+- Supported LoRa development board (e.g., Heltec, RAK, Seeed, ESP32, nRF52840)
+- LoRa radio module (LR11xx or compatible)
+- USB or UART connection to host
+
+## Building the Firmware
+
+### Prerequisites
+- [Zephyr SDK](https://docs.zephyrproject.org/latest/develop/getting_started/index.html)
+- [west](https://docs.zephyrproject.org/latest/develop/west/index.html) (Zephyr meta-tool)
+- Board support package for your target board
+
+### Steps
+1. **Initialize Zephyr workspace (if not already):**
+   ```sh
+   west init -l .
+   west update
+   west zephyr-export
+   ```
+2. **Build for your board:**
+   Replace `<board>` with your board's Zephyr name (e.g., `heltec_wifi_lora32_v2`).
+   ```sh
+   west build -b <board> .
+   ```
+   Example:
+   ```sh
+   west build -b heltec_wifi_lora32_v2 .
+   ```
+
+## Flashing the Firmware
+
+1. **Connect your board via USB.**
+2. **Flash using west:**
+   ```sh
+   west flash
+   ```
+   (You may need to specify `--runner` or additional options depending on your board.)
+
+## Serial Interface
+- Default baud rate: 115200
+- KISS protocol framing
+- Connect to `/dev/ttyUSBx` or equivalent on your host
+
+## Directory Structure
+- `app/`      – Zephyr application code
+- `drivers/`  – LoRa radio drivers (e.g., LR11xx)
+- `boards/`   – Board overlays and configuration
+- `dts/`      – Device tree sources
+- `zephyr/`   – Zephyr RTOS (west workspace)
+
+## License
+See LICENSE file for details.
+
+## References
+- [KISS TNC Protocol](https://en.wikipedia.org/wiki/KISS_(TNC))
+- [Zephyr Project Documentation](https://docs.zephyrproject.org/)