Replace bach’s UDP simulation backend with a first-pass turmoil-net adapter

[Copilot]

This PR swaps bach’s internal network backend over to turmoil-net as a first pass, with the goal of preserving the existing bach::net surface and surfacing the remaining compatibility gaps early. The current pass keeps existing UDP-centric behavior working while leaving a few advanced backend hooks intentionally stubbed or unsupported.

• Backend swap

  • Replaces the custom UDP socket backend with a turmoil-net-backed adapter under the existing bach::net API.
  • Keeps UdpSocket::{bind, connect, send, send_to, recv, recv_from} semantics intact for current callers.
  • Routes per-group execution through a shared turmoil_net::Net topology.

• Compatibility layer

  • Preserves bach hostname/group resolution so existing tests and call sites continue to use names like "server:8080".
  • Adapts turmoil-net packets into bach’s monitor packet shape so net-monitor callbacks continue to observe send/receive traffic.
  • Emulates GSO-style segmented sends by splitting payloads on SendOptions.segment_len.

• Runtime / scope hardening

  • Fixes scope restoration so thread-local runtime state is unwound correctly across panic paths.
  • Avoids aborts during monitor-triggered panics by ensuring cleanup does not re-enter torn-down TLS state.

• Intentional first-pass gaps

  • with_pcap_dir remains as a compatibility no-op for now.
  • Custom net queue allocators remain API-compatible but are not wired into the new backend yet.
  • Some advanced socket options remain unsupported by the adapter.
  • Adding new groups after the turmoil topology is initialized is not supported in this pass.

Example of the preserved public API shape:

use bach::net::UdpSocket;

async {
    let socket = UdpSocket::bind("0.0.0.0:0").await.unwrap();
    socket.send_to(b"ping", "server:8080").await.unwrap();

    let mut buf = [0; 4];
    let (len, addr) = socket.recv_from(&mut buf).await.unwrap();
    assert_eq!((&buf[..len], addr.port()), (b"pong", 8080));
};