Transport abstraction

TL;DR. The wire codec sits behind an async Transport trait on both ends. v0.1 ships only UnixSocketTransport, but no domain module names a concrete transport — that’s load-bearing for the QUIC and SSH-stdio implementations that ADR-0007 keeps purely additive.

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The wire codec sits behind an async Transport trait on both server and client. v0.1 ships exactly one implementation — UnixSocketTransport — but no domain module in phux-server or phux-client names a concrete transport type. All I/O goes through the trait.

This is a load-bearing invariant for ADR-0007 (Mosh-class transport and satellite forward-compat). It exists to keep two v0.2+ features purely additive:

• QUIC transport (via quinn) — provides connection migration, 0-RTT resumption, and TLS, giving us the UX properties of Mosh without reimplementing SSP.
• SSH-stdio transport — frames the wire codec over a child SSH process’s stdin/stdout, used by hub servers to reach satellites over existing SSH paths.

See ADR-0007 for the full forward-compat invariants (URI-shaped session IDs, hub-and-spoke satellite topology, per-pane encoder isolation).