Add client reconnect, Happy Eyeballs, and clean disconnect handling

[guyte149]

Reliability and UX overhaul of the client/server lifecycle. Previously the client exited on the first disconnect, blocked v6-first DNS against v4-only servers for the full QUIC handshake timeout, and the server silently leaked reverse-tunnel listeners when clients went away. Now:

• Client reconnects automatically with exponential backoff (200 ms → 300 s by default, configurable via --max-retry-count and --max-retry-interval, mirroring chisel). The same loop covers initial connect failures so clients started before the server is up keep retrying until it appears.
• Client races every resolved address with RFC 8305 Happy Eyeballs v2: parse_server_addr collects all candidates and reorders them by alternating address families, the client maintains one quinn endpoint per family lazily, and connect attempts are staggered by the spec-recommended 250 ms Connection Attempt Delay. Fixes the common "macOS localhost resolves to ::1 first, server is on 0.0.0.0" failure mode that previously blocked for ~30 s.
• Server installs a ^C handler that gracefully closes the QUIC endpoint with reason "server received ^C", so connected clients log the close immediately and proceed into the reconnect loop instead of waiting out the idle timeout.
• Server's per-connection accept loop decodes every ConnectionError variant into a human-readable INFO log; previously even a clean client shutdown produced zero server-side output (the message was at debug!).
• Server scopes per-tunnel work to a tokio::task::JoinSet whose shutdown().await fires the moment the connection ends, releasing reverse-tunnel listener ports immediately. New regression test in tests/disconnect_cleanup.rs asserts the listener is rebindable within ~600 ms of a client connection.close().
• Client briefly waits for quinn to flush the CONNECTION_CLOSE frame before tearing down the endpoint on shutdown, so the server reliably sees the close instead of racing wait_idle.

Public API changes:

• ClientConfig gains a reconnect: ReconnectConfig field.
• ServerEndpoint.addr (SocketAddr) is now addrs (Vec) plus a primary() accessor.
• create_client_endpoint takes the resolved server SocketAddr so it can pick the matching bind family.

Tests:

• tests/reconnect.rs covers initial-connect-before-server-up and server-restart-mid-session scenarios.
• tests/disconnect_cleanup.rs is the JoinSet-abort regression.
• Three unit tests in main.rs for the address-family interleaving.