test: exhaustive COBS interop tests against corncobs, porting guide, README

- 8 new interop tests cross-validating our COBS encoder/decoder against
  corncobs (including 500 random payloads). Total: 39 tests.
- New: src/board/PORTING.md — step-by-step guide to adding a new board
- New: crates/donglora-cobs/README.md — documents design choice and testing
- Updated README with lora_bridge.py example

src/board/PORTING.md

@@ -0,0 +1,86 @@
+# Porting DongLoRa to a New Board
+
+## Steps
+
+### 1. Create the board file
+
+Create `src/board/your_board.rs`. Use an existing board as a template:
+- ESP32-S3 boards: copy `heltec_v4.rs`
+- nRF52840 boards: copy `rak_wisblock_4631.rs`
+
+### 2. Define concrete types
+
+Your board must export these type aliases:
+
+```rust
+pub type RadioDriver = ...; // Must work with lora_phy::LoRa<RadioDriver, Delay>
+pub type UsbDriver = ...;   // Must implement embassy_usb_driver::Driver<'static>
+pub type DisplayI2c = ...;  // Must implement embedded_hal_async::i2c::I2c
+```
+
+### 3. Define peripheral bundles
+
+```rust
+pub struct RadioParts {
+    pub driver: RadioDriver,
+    pub delay: embassy_time::Delay,
+}
+
+pub struct UsbParts {
+    pub driver: UsbDriver,
+}
+
+pub struct DisplayParts {
+    pub i2c: DisplayI2c,
+    pub mac: [u8; 6],
+}
+```
+
+### 4. Implement the LoRaBoard trait
+
+```rust
+use super::traits::LoRaBoard;
+
+impl LoRaBoard for Board {
+    const NAME: &'static str = "Your Board Name";
+    const TX_POWER_RANGE: (i8, i8) = (-9, 22); // check your radio + PA
+    fn init() -> Self { ... }
+    fn mac_address() -> [u8; 6] { ... } // read from efuse, FICR, etc.
+}
+```
+
+The compiler will refuse to build if any of these are missing.
+
+### 5. Implement `Board::into_parts()`
+
+```rust
+impl Board {
+    pub fn into_parts(self) -> (RadioParts, UsbParts, Option<DisplayParts>) {
+        // Initialize SPI, DMA, radio, USB, I2C
+        // Return peripheral bundles
+    }
+}
+```
+
+### 6. Add Cargo feature
+
+In `Cargo.toml`, add a feature with your board's HAL dependencies:
+
+```toml
+[features]
+your_board = ["dep:your-hal", ...]
+```
+
+### 7. Add to Justfile
+
+Add your board's feature/target/chip to the `boards` list at the top of `Justfile`.
+
+### 8. Build and test
+
+```sh
+just check your_board    # Must compile
+just clippy your_board   # Must be clean
+just build your_board    # Produces firmware
+```
+
+`build.rs` auto-discovers your `.rs` file — no template editing needed.