Add BitChat v2 source routing

[callebtc]

Summary

• add BitChat packet v2 parsing and encoding with a 4-byte payload length, routed signatures, and v1 compatibility
• advertise verified direct neighbors and maintain bounded direct-peer and topology tables
• compute confirmed-edge shortest paths, attach source routes to directed traffic, and relay toward the next hop with loop rejection and flood fallback
• carry routed packets over the existing LoRa trunk and document the routing behavior

Scope

This branch is based directly on upstream main and isolates the routing and packet-v2 functionality. It contains no additional board-support, display, or LoRa-reliability changes.

Testing

• ESP-IDF 6.0 build for ESP32-S3
• ESP-IDF 6.0 build for ESP32-C3
• boot-time packet-codec self-test covers v1 and v2 source-route round trips