Transport abstraction layer for the Offline Protocol SDK.

This crate defines the [Transport] trait and the transport state machines for BLE, Wi-Fi Direct, Internet, Nostr, and Reticulum.

This crate performs no network or radio I/O

No sockets are opened and no radios are touched here. Each transport is a thread-safe protocol/queue engine — status, send/receive queues, metrics, and (for BLE) fragmentation state — driven from two sides:

• The protocol engine talks to the [Transport] trait: send() enqueues an outbound message, receive() dequeues an inbound one.
• A platform bridge (Swift/Kotlin via the UniFFI bindings, or any host program) owns the actual sockets, relays, and radios. It drains the outbound queue (get_next_message() / get_next_signed_event() / get_next_fragment()), performs the real I/O, reports the outcome (confirm_sent() / report_send_failure()), injects inbound bytes (on_data_received() / on_fragment_received()), and drives availability (on_status_changed()).

Without a platform bridge attached, send() queues messages that never go anywhere — and, BLE excepted, start() never makes a transport Available in the first place, so send() returns TransportNotAvailable. A pure-Rust consumer must play the bridge role itself: mark the transport available, drain the queues, and do its own I/O.

Most transports signal pending outbound work through a set_on_messages_available callback instead of requiring a poll timer; [InternetTransport] is the exception and must be polled.

All code in this crate is 100% safe Rust with no unsafe blocks.