README: say what a dead connection does to a library caller

Chunk 29 changed the contract for the seven writes the device never
acks: they can now throw where they previously could not, because
send() on a closed socket discards silently rather than throwing. The
JSDoc and PBZ-PLAN said so; the README, which is where a library
consumer actually looks, did not.

Also records that detection needs traffic, so a parked reader notices
later than a request/response method does.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PZnUXRsLLnt2A7pR3X4Luj