fix(pair): say why Astation's connection closed during verification

[guohai]

Summary

atem pair failed with "Astation connection closed during device verification" after the safety codes matched, with no way to tell why: the WebSocket reader dropped the close frame's reason and silently dropped any Astation message it couldn't parse.

• The reader records how the socket ended (close code + reason, a transport error, or no close frame) and the type of each message that didn't parse (never its content; at most 8 per socket).
• atem pair appends both to the error, e.g. Astation connection closed during device verification (closed by Astation: 1008 "signing failed"; unreadable messages from Astation: deviceVerified).

Heartbeats were considered and left out: Atem doesn't send them anywhere today, and long-lived TUI connections stay up, so an idle close is unlikely.

Testing

• New test: a fake Astation sends an unparseable deviceVerified then closes with 1008 "signing failed"; the error names both.
• cargo test: 1266 unit + agent_launch_binary 3, key_agent_binary 2, terminal_ui_binary 2, all pass. Clippy: no new warnings.

Generated with SMT smt@agora.build

[guohai]

Dropped: under the approved session-multiplexing design, atem pair no longer reconnects an already paired profile, and connection ownership moves to the shared runtime. Close reasons (incl. the relay's replaced-connection code) will be captured there as part of the runtime's transport handling instead of as a pair-only diagnostic.

Generated with SMT smt@agora.build