Add transport key to packet and utilities for mesh packet calcs

[rightup]

Add transport codes to the packet class and new functions to work out transport code with packet data.

[Copilot]

Pull request overview

This PR adds transport key functionality to the mesh packet system, enabling packet authentication using region-based transport codes. The implementation matches the C++ MeshCore firmware protocol, ensuring cross-platform compatibility.

Key Changes:

• Introduced transport key generation and transport code calculation utilities
• Extended packet structure to support optional 4-byte transport code fields for TRANSPORT_FLOOD and TRANSPORT_DIRECT route types
• Added helper methods to identify packet routing characteristics

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
src/pymc_core/protocol/transport_keys.py	New module providing get_auto_key_for() to generate 128-bit keys from region names and calc_transport_code() to compute HMAC-based transport codes for packet authentication
src/pymc_core/protocol/packet.py	Extended Packet class with transport_codes attribute, updated serialization/deserialization to conditionally include 4-byte transport codes, added routing helper methods (has_transport_codes(), is_route_flood(), is_route_direct()), and updated documentation to reflect transport code support

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[Copilot]

The packet parameter is missing a type annotation. Based on usage in the function body (calling packet.get_payload_type() and packet.get_payload()), it should be typed as Packet from the .packet module. This would improve type safety and IDE support.

Suggestion: Add type annotation like packet: "Packet" or import Packet and use packet: Packet.

[Copilot]

@rightup I've opened a new pull request, #17, to work on those changes. Once the pull request is ready, I'll request review from you.