refactor(tui): extract event stream connection

[jlongster]

Summary

• extract one event stream connection attempt from the reconnect loop
• pass only the stream AbortSignal into the extracted function
• separate request setup, handshake, connected-state publication, event forwarding, and cleanup
• leave reconnection, backoff, and service resolution in start()

Behavioral equivalence

• run the same 16-test characterization harness against untouched origin/v2 and this branch
• cover 20 behavior areas including handshake ordering, delivery, logs, errors, timeout, reconnects, cleanup, cancellation, service replacement, listener failures, malformed SSE, and attempt reset
• produce byte-identical 828-line, 17,399-byte transcripts for both implementations
• matching SHA-256: 561bd5a1e22eca7c9821b293920308c4487fadc9751b0ca4a6e41cd3f21bb36b

Testing

• bun test test/cli/tui/client-connection-characterization.test.tsx
• bun test test/cli/tui/use-event.test.tsx
• bun typecheck