feat(bssh): verify known host keys

[ryanbreen]

Summary

• exposes server host-key identity from client KEX
• adds bssh known_hosts-style verification at /tmp/bssh_known_hosts by default, with --known-hosts override
• TOFU-pins unknown hosts, silently accepts matching pinned hosts, and refuses mismatched pins before auth/channel setup

Validation

• cargo check --manifest-path libs/libbreenix/Cargo.toml --features std
• userspace/programs/build.sh --arch aarch64
• cargo build --release --features testing,external_test_bins --bin qemu-uefi (warning scan empty)
• Parallels proof via Breenix bsshd exec into bssh targeting operator Mac sshd 10.0.1.210:22:
  • Case A first connect: empty known_hosts, pinned [10.0.1.210]:22, returned Darwin, exit 0
  • Case B strict reverify: reused the pinned key without re-pinning, returned Darwin, exit 0
  • Case C tampered pin: wrong Breenix key pinned for [10.0.1.210]:22, bssh refused with host key verification failed, no BSSH_EXEC_BEGIN, exit 1

Artifacts: /Users/wrb/Downloads/Ralph/breenix-interrupt-io-roadmap-1780056222/turn93-artifacts