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/from_body.rs

@@ -0,0 +1,62 @@
+#![no_main]
+//! Fuzz `protocol::from_body` (bincode deserialization) for every protocol and
+//! native struct that is decoded from untrusted wire bytes. None may panic.
+
+use libfuzzer_sys::fuzz_target;
+
+use dosh::auth::{AttachTicketPlain, BootstrapResponse, SealedAttachTicket};
+use dosh::native::{
+    HostPublicKey, NativeAuthOk, NativeClientHello, NativeServerHello, NativeUserAuth,
+};
+use dosh::protocol::{
+    self, AttachOk, AttachReject, BootstrapAttachRequest, Frame, Input, NativeAuthCheckOkBody,
+    NativeAuthOkBody, NativeClientHelloBody, NativeServerHelloBody, NativeUserAuthBody, Resize,
+    ResumeRequest, StreamClose, StreamData, StreamEof, StreamOpen, StreamOpenOk, StreamOpenReject,
+    StreamWindowAdjust, TicketAttachBody, TicketAttachEnvelope, TicketAttachOkEnvelope,
+};
+
+macro_rules! try_body {
+    ($data:expr, $ty:ty) => {
+        let _ = protocol::from_body::<$ty>($data);
+    };
+}
+
+fuzz_target!(|data: &[u8]| {
+    // protocol.rs structs
+    try_body!(data, BootstrapAttachRequest);
+    try_body!(data, TicketAttachEnvelope);
+    try_body!(data, TicketAttachBody);
+    try_body!(data, TicketAttachOkEnvelope);
+    try_body!(data, AttachOk);
+    try_body!(data, AttachReject);
+    try_body!(data, ResumeRequest);
+    try_body!(data, Input);
+    try_body!(data, Resize);
+    try_body!(data, Frame);
+    try_body!(data, StreamOpen);
+    try_body!(data, StreamOpenOk);
+    try_body!(data, StreamOpenReject);
+    try_body!(data, StreamData);
+    try_body!(data, StreamWindowAdjust);
+    try_body!(data, StreamEof);
+    try_body!(data, StreamClose);
+
+    // native handshake wrapper bodies
+    try_body!(data, NativeClientHelloBody);
+    try_body!(data, NativeServerHelloBody);
+    try_body!(data, NativeUserAuthBody);
+    try_body!(data, NativeAuthOkBody);
+    try_body!(data, NativeAuthCheckOkBody);
+
+    // bare native handshake structs
+    try_body!(data, NativeClientHello);
+    try_body!(data, NativeServerHello);
+    try_body!(data, NativeUserAuth);
+    try_body!(data, NativeAuthOk);
+    try_body!(data, HostPublicKey);
+
+    // auth.rs structs
+    try_body!(data, BootstrapResponse);
+    try_body!(data, SealedAttachTicket);
+    try_body!(data, AttachTicketPlain);
+});