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

crates/donglora-cobs/README.md

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+# donglora-cobs
+
+Minimal COBS (Consistent Overhead Byte Stuffing) encoder/decoder.
+
+- `no_std`, zero-alloc, zero dependencies at runtime
+- Spec-compliant: implements Cheshire & Baker (IEEE/ACM 1999)
+- Exhaustively tested: 39 tests including cross-validation against `corncobs`
+
+## Why our own crate?
+
+DongLoRa is an embedded firmware project. We want:
+- Zero runtime dependencies (just 130 lines of code)
+- Complete control over the implementation
+- Spec compliance verified against both the canonical Wikipedia vectors
+  AND a battle-tested third-party crate (`corncobs`)
+
+For higher-throughput use cases, consider
+[corncobs](https://crates.io/crates/corncobs) which has been
+benchmark-optimized for streaming applications.
+
+## Testing
+
+```sh
+cargo test -p donglora-cobs
+```
+
+Tests include:
+- 18 canonical COBS vectors (Wikipedia / IEEE/ACM spec)
+- Round-trip verification for various payload sizes
+- 8 interop tests cross-validating against `corncobs` (dev-dependency)
+- 500 randomized round-trip payloads verified against `corncobs`