feat(ingestion): show stash dwell and retry timing in batch flow graph

[jose-sequeira]

Problem

Follow-up to #71033. In the consumer debug batch detail view, a deferred batch only shows that work was stashed ("flushed & re-routed" or "awaiting flush"). The events carry timestamps, but the flow graph drops them when aggregating, so you can't see when the work was deferred, how long it sat stashed, or when and where it was retried without diffing lifecycle timestamps by hand.

Changes

• The consumer never emitted the DeferredFlushed debug event: flush_deferred built the re-routed sub-batches but never recorded the event, so the UI showed every stashed batch as "awaiting flush" forever. The flush path now records it (with a dispatcher regression test).
• The batch model now keeps ts_ms on deferred, flushed, retry, and exhausted entries instead of discarding it.
• The stash node in the flow graph shows when the work was deferred and, once flushed, the dwell time ("flushed after 340ms"); until then it still shows "awaiting flush".
• Stash-to-worker flush edges are labeled with the flushed message count plus dwell relative to the first deferral ("flushed 120 · +340ms").
• Assign-to-worker edges show send retry counts ("sent 120 · ↻2") and turn amber when retries happened.
• The lifecycle timeline annotates each flush event with how long the stashed work waited.
• Adds a small fmtDur helper (ms / s / m formatting).

How did you test this code?

Browser-only vanilla JS with no test harness. I extracted the inline script and ran node --check for syntax, plus a node unit test of buildFlow with a synthetic batch (deferred at t, flushed at t+340ms to one worker, one retry on another) asserting the stash node dwell text, the flush edge label, the retry edge label and color, the fmtDur formats, and the "awaiting flush" state for a still-stashed batch. I was not able to exercise it against a live consumer in this session.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

Docs update

Internal debug tooling, no user-facing docs affected.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Authored with Claude Code, directed by José: the deferred/stash graphics didn't show how and when stashed work was retried. I (Claude) threaded the existing event timestamps through the client-side batch model rather than adding new backend events, since the SSE stream already carries ts_ms on every event. Dwell on flush edges is measured against the first deferral of the batch, which is an approximation when a batch defers multiple times — the lifecycle timeline has the exact per-event gaps.

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Deploy status

Environment	Status	Deployed At	Workflow
dev	✅ Deployed	2026-07-15 16:00 UTC	Run
prod-us	✅ Deployed	2026-07-15 16:19 UTC	Run
prod-eu	✅ Deployed	2026-07-15 16:21 UTC	Run