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

CLAUDE.md

@@ -0,0 +1,23 @@
+# LoRa KISS modem
+
+## Hardware Platform
+- Common LoRa development boards from vendors like Heltec, RAK, Seeed
+- Typical MCU's include: ESP32, nRF52840
+
+## Framework
+- Zephyr RTOS
+
+## Memory Budget
+- Application code: Max 256KB (leave 768KB for bootloader updates)
+- RAM usage: Max 64KB (leave 64KB safety margin)
+- Stack: 4KB allocated
+- Heap: NONE (no dynamic allocation)
+
+## Testing Strategy
+- Unit tests run on host (arm64 / x86) with mocked HAL
+- Integration tests run on target hardware with automated test harness
+
+## Conventions
+- Overlay filenames: app.overlay or boards/<BOARD>.overlay
+- Kconfig: prj.conf (shared), boards/<BOARD>.conf (board-specific)
+- After .conf edits, verify against build/zephyr/.config