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refactor: use sync.Cond for jsonPipe queue; support multiple consumers

Address review feedback on the deadlock fix:
- Replace the hand-rolled buffered-signal-channel + wake() condvar with
  sync.Cond over the existing mutex. This is the idiomatic primitive for
  'wait until queue non-empty or closed', removes the load-bearing
  non-blocking-send/re-check-loop subtlety, and drops ~15 lines.
- Broadcast instead of a single-token signal, so multiple concurrent
  Poll() consumers are supported (the old single-token design would have
  stranded messages if a second consumer were ever added).
- Consolidate the triplicated deadlock rationale into one comment on
  enqueue (the invariant owner).
- Rework TestJsonPipeEnqueueNeverBlocks to also drain and assert ordering
  after the burst, and add TestJsonPipeMultipleConsumers to cover the
  exactly-once delivery guarantee.