dp: add wake watchdog and trace hooks

[fergusfinn]

Summary

• Re-publish DP request counts after a request is handled while the engine is idle,
  so a paused engine drained by aborts reports its zeroed counts without waiting for
  a model step (_publish_idle_state hook).
• Add a DP wake watchdog in the coordinator: when unfinished work is visible while
  engines are believed paused (or the running belief has gone stale), re-broadcast
  the idempotent START_DP_WAVE, at most every 10 s.
• Engines publish an explicit engines_paused marker when entering the paused state
  so the coordinator can tell "asleep" from "busy with constant counts".
• Add opt-in VLLM_DP_TRACE=1 logs for wave starts, finish-sync votes, engine
  sleep, external finishes, and short finishes (diagnostics only).

Classification

Robustness safety-net + diagnostics.

Problem

The DP wave protocol has message races (e.g. a front-end new-request notification
crossing a wave_complete) in which work can be stranded on a paused engine with
nothing in the protocol retrying. Separately, a DP engine drained by aborts while
paused never republishes its zeroed counts, so the load balancer keeps avoiding it.

Validation

The watchdog and idle-republish are safety nets for non-deterministic lost-wakeup
races; a deterministic hang/drain serve reproduction was not achieved in the
available time. What was verified: all five changed modules parse; the new
SchedulerStats.engines_paused field is added with a backward-compatible default;
the watchdog re-broadcast uses START_DP_WAVE, which is idempotent for already
-running engines, is rate-limited to 10 s, and only fires when work is visibly
stranded — so the safety net cannot disrupt a healthy run. The VLLM_DP_TRACE
half is diagnostics-only and emits the events it documents.

Verdict

low-risk robustness safety-net (mechanism sound; deterministic repro not
achieved). The re-broadcast is idempotent and bounded, so it is safe to carry;
the trace half is opt-in diagnostics. Keep, with the caveat that the specific
lost-wakeup race it guards was not reproduced on demand.

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