mDNS-based peer discovery, QUIC transport, and Python bindings.

What's included:
- Stable LAN peer discovery and announce via mDNS
- QUIC envelope transport with Ed25519-signed certs
- dial-back reply path fix for request/response exchanges (#21)
- Per-crate READMEs (#20)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>