PBZ-PLAN: record what the transport cannot see

Item 19: a peer that goes silent with TCP up is invisible — writes
return success and go nowhere, and only an outbound liveness probe
would catch it. Measured, and it is the most likely failure this
hardware has.

Item 20: undici has no timeouts of its own. Every timeout in this
system is one pbz wrote, which is worth knowing before someone removes
one for looking redundant.

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