feat: add Waveshare RP2040-LoRa board support

First RP2040 board and third MCU family (after ESP32-S3 and nRF52840).

Hardware: RP2040 + SX1262 radio, 850-930 MHz HF, PE4259 antenna switch,
LED on GPIO25, no OLED, USB-C via FPC ribbon cable to adapter board.

New files:
- src/hal/rp2040.rs: MCU primitives (SPI bus, USB driver, flash unique ID MAC)
- src/board/waveshare_rp2040_lora.rs: board implementation
- ld/rp2040-memory.x: linker memory layout

Also fixes cortex-m-rt/set-vtor (moved from global dep to per-board feature,
since Cortex-M0+ has no VTOR register) and makes build.rs memory.x selection
target-aware.

CHANGELOG.md

@@ -5,6 +5,22 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/),
 and this project adheres to [Semantic Versioning](https://semver.org/).
 
+## [0.3.0] - 2026-04-10
+
+### Added
+
+- Waveshare RP2040-LoRa board support (RP2040 + SX1262, no OLED, LED on GPIO25)
+  - First RP2040 board — third MCU family after ESP32-S3 and nRF52840
+  - New `hal/rp2040.rs` module with MCU primitives (SPI, USB, flash unique ID)
+  - USB-C via FPC ribbon cable to adapter board
+  - PE4259 antenna switch on GPIO17, 850-930 MHz (HF)
+
+### Changed
+
+- `cortex-m-rt/set-vtor` moved from global dependency to per-board feature
+  (Cortex-M0+ has no VTOR register)
+- `build.rs` memory.x selection is now target-aware (thumbv6m vs thumbv7em)
+
 ## [0.2.1] - 2026-04-07
 
 ### Fixed