feat(krun): add exit signal handlers, exit handle, and custom console backends

[appcypher]

Summary

• Add cross-platform SIGTERM/SIGUSR1 signal handlers that trigger graceful VMM shutdown through the exit event fd, ensuring exit observers run before process termination
• Add ExitHandle type for triggering VM exit programmatically from any thread (cloneable, thread-safe, async-signal-safe)
• Propagate the VM exit code through the observer chain so on_exit callbacks receive the i32 exit code, and expose Vm::exit_code() as a shared Arc<AtomicI32>
• Add ConsolePortBackend trait and PortConfig::Custom variant for custom in-process console port I/O without file descriptors on the data path

Changes

• New src/vmm/src/exit_signal.rs: POSIX sigaction-based handlers for SIGTERM and SIGUSR1 that write to the VMM exit event fd, working on both Linux and macOS
• New src/krun/src/api/exit_handle.rs: ExitHandle struct that dups the exit event fd write end and exposes a trigger() method, with Clone/Send/Sync implementations
• New src/krun/src/backends/console.rs: Re-exports ConsolePortBackend from the devices crate
• Modified src/devices/src/virtio/console/port_io.rs: Added ConsolePortBackend trait with read/write/read_wake_fd methods, plus ConsolePortBackendInputAdapter and ConsolePortBackendOutputAdapter that bridge the trait to PortInput/PortOutput
• Modified src/devices/src/virtio/device.rs: Changed VmmExitObserver::on_vmm_exit() and its blanket Fn impl to accept exit_code: i32
• Modified src/krun/src/api/builder.rs: on_exit callback signature changed from Fn() to Fn(i32), pre-creates exit_evt and exit_code and passes them to Vm::new()
• Modified src/krun/src/api/builders.rs: Added ConsoleBuilder::custom() method that wraps a ConsolePortBackend into PortConfig::Custom
• Modified src/krun/src/api/vm.rs: Added Vm::exit_handle() and Vm::exit_code() public methods; updated enter() to pass exit_evt and exit_code to build_microvm
• Modified src/vmm/src/builder.rs: build_microvm now accepts pre-created exit_evt: EventFd and exit_code: Arc<AtomicI32> instead of creating them internally; registers exit signal handlers; added PortConfig::Custom arm in create_explicit_ports; attach_console_devices takes owned VirtioConsoleConfigMode instead of a reference
• Modified src/vmm/src/lib.rs: notify_exit_observers and stop propagate exit_code: i32
• Modified src/vmm/src/resources.rs: Added PortConfig::Custom variant with boxed PortInput/PortOutput; removed Debug/Clone derives from PortConfig
• Modified src/libkrun/src/lib.rs: Updated krun_start_enter to create and pass exit_evt/exit_code to build_microvm
• Updated examples/rust_vm/src/main.rs: on_exit callback now receives and prints the exit code

Test Plan

• Run cargo build to verify compilation succeeds across all crates
• Run cargo test to verify existing unit tests pass (including updated Vm::new() signatures in vm.rs tests)
• Verify the rust_vm example compiles: cargo build -p rust_vm
• Test graceful shutdown by sending SIGTERM to a running VM and confirming exit observers fire
• Test ExitHandle::trigger() from a background thread to verify programmatic shutdown works