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/known_hosts.rs

@@ -0,0 +1,14 @@
+#![no_main]
+//! Fuzz the known_hosts parser and the host-public-key line parser. None may
+//! panic on arbitrary input.
+
+use libfuzzer_sys::fuzz_target;
+
+use dosh::native::{parse_host_public_key_line, parse_known_hosts};
+
+fuzz_target!(|data: &[u8]| {
+    if let Ok(text) = std::str::from_utf8(data) {
+        let _ = parse_known_hosts(text);
+        let _ = parse_host_public_key_line(text);
+    }
+});