subagent_panel: coalesce drain can deliver stream events out of order at stream end

Bug

subagent_panel/coalesce_patch.py — lines 101–112 (_drain_pending_task) vs 129–144 (flush_pending_callbacks).

If the 50 ms drain fires just before a sub-agent stream ends:

1. _drain_pending_task moves the pending events into a batch and calls loop.create_task(_fire_batch(batch)) (line 112) — the batch is scheduled, not executed.
2. The stream wrapper's finally then calls flush_pending_callbacks(), which drains any newer events and runs them synchronously via await _fire_batch(batch) (line 144).

The newer events therefore reach the panel before the loop gets around to the earlier scheduled batch — part_end/completion data for the tail of a stream can render before the earlier part_start/text of the same stream.

Runtime consequence

The sub-agent panel tree can repaint out of order (a finished row briefly appearing before its content) — flapping on exactly the busy-swarm scenes this patch was written for.

Suggested fix

Never leave the timer's batch as a deferred task — route it through the same flush path as stream-end, or make flush_pending_callbacks cancel/absorb a scheduled drain before firing.

Read subagent_panel/coalesce_patch.py, especially _drain_pending_task and flush_pending_callbacks at the cited lines, and trace how the stream wrapper calls the flush path in finally. Verify that a timer-drained batch is delivered before newer stream-end events, with part_start and text preceding part_end or completion data.