Add hostile-network tests, parser fuzzing, and CI fuzz job

Track B (milestone 5 / §16 hardening):

- tests/hostile_network.rs: in-process UDP relay/shim between a test
  client and a real dosh-server that can drop, reorder, and duplicate
  datagrams, and rebind its upstream socket mid-session. Asserts:
  session survives loss/reorder, duplicated/replayed Input is applied at
  most once, stale packets after resume are ignored (not fatal), and a
  client source-address change preserves the session.
- tests/parser_robustness.rs: deterministic randomized tests throwing
  garbage at every reachable public parser (packet decode, from_body for
  all protocol/native structs, authorized_keys, known_hosts, host-key
  line, ssh-ed25519 blob, bootstrap, attach ticket); asserts none panic.
- fuzz/: standalone cargo-fuzz crate (own [workspace], non-default
  member) with libfuzzer-sys harnesses for packet decode, from_body,
  authorized_keys, known_hosts, handshake structs+verifiers, and attach
  tickets. README documents the run command.
- .github/workflows/ci.yml: add fuzz-smoke job (nightly + cargo-fuzz,
  short -max_total_time per target, tolerant if tooling unavailable);
  existing fmt/test/build/bench steps unchanged.

cargo fmt --check and cargo test are green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

fuzz/fuzz_targets/handshake_structs.rs

@@ -0,0 +1,44 @@
+#![no_main]
+//! Fuzz the native handshake structs and their structural verifiers.
+//!
+//! Beyond plain deserialization (covered by the `from_body` target), this drives
+//! the verifier state machine: if the input happens to decode into the handshake
+//! structs, run `verify_server_hello`, `user_auth_transcript`, and
+//! `verify_native_user_auth` on them. These run on attacker-controlled material
+//! during the handshake and must reject (Err) without panicking.
+
+use libfuzzer_sys::fuzz_target;
+
+use dosh::native::{
+    NativeClientHello, NativeServerHello, NativeUserAuth, user_auth_transcript,
+    verify_native_user_auth, verify_server_hello,
+};
+use dosh::protocol;
+
+fuzz_target!(|data: &[u8]| {
+    // Split the input into three slices and try to decode each into a handshake
+    // struct. Use a length prefix scheme that is robust to short inputs.
+    if data.len() < 3 {
+        return;
+    }
+    let n = data.len();
+    let a = n / 3;
+    let b = 2 * n / 3;
+    let (chunk_client, chunk_server, chunk_auth) = (&data[..a], &data[a..b], &data[b..]);
+
+    let client: Option<NativeClientHello> = protocol::from_body(chunk_client).ok();
+    let server: Option<NativeServerHello> = protocol::from_body(chunk_server).ok();
+    let auth: Option<NativeUserAuth> = protocol::from_body(chunk_auth).ok();
+
+    if let (Some(client), Some(server)) = (&client, &server) {
+        // Host signature verification over the transcript must not panic.
+        let _ = verify_server_hello(client, server);
+
+        if let Some(auth) = &auth {
+            // Transcript construction and full user-auth verification (signature
+            // check + authorized-key matching) must not panic on garbage.
+            let _ = user_auth_transcript(client, server, auth);
+            let _ = verify_native_user_auth(client, server, auth, &[], None);
+        }
+    }
+});