Update region design doc with upstream source analysis and implementation status

Corrected from web-scraped info to actual MeshCore source code analysis:
- Transport codes are uint16_t[2] (4B on wire), not UTF-8 strings
- Codes are cryptographic: HMAC-SHA256(SHA256(name), payload) -> 2 bytes
- Added wire format, serialization code, and findMatch() logic
- Referenced actual source files (Packet.h/cpp, RegionMap.h/cpp,
  TransportKeyStore.h/cpp, MyMesh.cpp)
- Documented Issue #1747 wildcard ambiguity
- Updated implementation phases: Phase 1-3 done, Phase 4 deferred

Author: atomozero