feat(tls): multi-peer mesh support for TLS transport (#136)

[vxfemboy]

Fixes #136: Implements full multi-peer support for transport = tls in yipd.

Changes Summary

1. Dynamic Multi-FD Epoll Primitive (crates/yip-io/src/epoll.rs):
   • Added safe dynamic multi-token epoll (create, add, remove, wait_tokens), preserving #![forbid(unsafe_code)] in yipd.
   • Maintained backward compatibility for quic.rs (new, wait).
2. Multi-Peer Connection Lifecycle & Role Resolution (bin/yipd/src/tls.rs):
   • Implemented PeerConn state machine (Disconnected, Connecting, Connected) with exponential reconnect backoff (100ms -> 5000ms).
   • Pairwise static-key role tiebreak (local_public < peer_public dials, local_public > peer_public listens) to prevent connection glare.
   • Enforced fail-closed INNER_HANDSHAKE_TIMEOUT = 10s deadline per peer.
3. Multi-Peer Event Loop & Egress Demux (bin/yipd/src/tls.rs):
   • Non-blocking pump loop handling TOKEN_TUN, TOKEN_LISTENER, and TOKEN_PEER_BASE + idx.
   • Ingress deframing into PeerManager::on_udp with inner confirmation gating, immediate egress reply forwarding, and TUN delivery.
   • Egress demuxing by d.dst across active streams.
   • Cadence tick routing keepalive and cover datagrams.
4. Config & Test Updates (bin/yipd/src/config.rs):
   • Removed single-peer TLS warning and updated config tests.
   • Added drive_multi_peer_concurrent_pump unit test verifying 2 concurrent loopback TLS peers with zero cross-talk or packet leaks.

Verification

• cargo test --all: All 351 yipd tests and 42 integration/netns tests passed cleanly.
• cargo clippy --all-targets -- -D warnings: 0 warnings.
• cargo fmt --check: formatting clean.
• Zero wire format or protocol changes.