Adopt bitchat-protocol (vendored), switch license to Unlicense

[BradWhittington]

Summary

• Adopt bitchat-protocol as a vendored in-tree dependency under src/python_bitchat_client/_vendor/bitchat_protocol (sourced from upstream v0.1.1), including provenance metadata and upstream license artifacts.
• Refactor src/python_bitchat_client/protocol.py to delegate packet/TLV encode-decode behavior to the vendored implementation while preserving existing public API contracts expected by this client library.
• Add and run vendored upstream protocol tests via pytest discovery (testpaths), plus local dependency-policy and compatibility coverage.
• Switch project licensing from MIT to The Unlicense and document license boundaries for vendored upstream code in README/CHANGELOG/AGENTS.
• Bump package version to 0.3.0 and include changelog entries for vendoring, testing, and licensing updates.

Verification

• uv run pytest -q (113 passed, 1 skipped)
• uv build (built python_bitchat_client-0.3.0 sdist and wheel)