test: expand coverage (graceful shutdown, pure fns, SOCKS5 TCP) + client auto-reconnect

[Itsusinn]

Overview

Expands test coverage across the workspace and adds a new configurable client auto-reconnect feature for the TUIC outbound (with tests). No production behavior changes other than the reconnect feature, which is on by default and backward-compatible.

What changed

Graceful shutdown

• Unit tests for the backend-agnostic server core: the acceptor_loop cancellation primitive (cancel mid-accept, pre-cancelled, benign close, timed-out, fatal error) plus is_tuic_prefix / read_prefix.
• Integration tests that the cancellation chain drains rather than hangs:
  • TUIC inbound listen loop + per-connection tasks drain on cancel (idle and with a live connection).
  • SOCKS5 inbound accept loop + in-flight session abort on cancel.
  • tuic-client TCP/UDP forwarders drain on cancel.

Pure-function coverage (filling gaps)

• wind-core: is_private_ip, StackPrefer::from_str, pick_addr_by_preference, filter_addrs_by_preference.
• wind-socks: parse_udp_request_sync (all ATYPs + malformed headers), unmap_v4_mapped, target_addr_to_socket, convert_addr/convert_to_socks_addr round-trips.
• wind-base (previously zero tests): resolve_target — IP-literal bypass and domain-via-resolver.

SOCKS5 TCP end-to-end

Dependency-free hand-rolled SOCKS5 client: no-auth and RFC 1929 password auth (correct/incorrect), IPv4 and domain CONNECT with echo round-trip, and the unsupported-command path.

Client auto-reconnect (new feature)

• TuicOutbound now stores its connection in an ArcSwap. A supervisor task runs one session (heartbeat + incoming handling) until the connection drops, then reconnects with exponential backoff and swaps the fresh connection in — handle_tcp/handle_udp transparently use the new connection. Connection loss is detected promptly via conn.closed().
• Shutdown is honored even mid-handshake (reconnect races the shutdown token).
• In-flight streams on the old connection are not resurrected; callers retry and get new streams (correct tunnel-reconnect semantics).
• Configurable via ReconnectConfig { enabled, initial_backoff, max_backoff }, wired from the client Relay config (reconnect, reconnect_initial_backoff, reconnect_max_backoff) with serde defaults — existing configs keep working unchanged.
• Tests: pure next_backoff, reconnect-after-server-restart, shutdown-while-reconnecting, reconnect-disabled, and config parsing/defaults.

Reviewer notes

• The quiche outbound is a builder stub (no live client path), so reconnect targets only the quinn client backend.
• Relay uses #[serde(default)] with per-field educe defaults, so the three new fields are backward-compatible; one Relay literal in tuic-tests that didn't use ..Default::default() was updated.
• Verified locally: full cargo build --workspace, cargo +nightly fmt --check, and cargo clippy --tests are clean (no new warnings). Test suites green, including the tuic-tests end-to-end integration suite (full client+server) and 5× stable reruns of the timing-sensitive reconnect test.

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