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/Cargo.toml

@@ -7,3 +7,7 @@ description = "COBS encoder/decoder — no_std, zero-alloc, spec-compliant"
 
 [features]
 default = []
+
+[dev-dependencies]
+corncobs = "0.1"
+rand = "0.8"