Coalesce chat stream rendering

[plux]

Context

• Follow-up to #320: Coalesce assistant stream rendering

Description

Chat stream buffering previously focused on top-level assistant text, but chat
history can also receive subagent text and toolCallPrepare UI updates while a
response is still streaming. Immediate lifecycle events can arrive between those
buffered updates, which can reorder history or allow stale prepare UI to
overwrite newer tool state.

This PR extends stream coalescing to the other chat render paths. It preserves
receive order across top-level assistant text, nested subagent output, and
prepare UI updates.

Changes

• Queue top-level assistant text, parent-scoped subagent text, and toolCallPrepare UI updates through the stream buffer.
• Flush queued content before immediate lifecycle boundaries so history renders in receive order.
• Keep running progress events from flushing pending text prematurely.
• Finalize pending prepare state on tool lifecycle events so stale prepare UI cannot overwrite completed tool state.
• Preserve existing subagent child output when parent lifecycle updates change the parent label or status.
• Capture parent text before nested child blocks so collapsed expandable blocks reopen in receive order.
• Add regression coverage for render ordering, prepare finalization, running progress events, and nested subagent streams.
• Add a benchmark fixture for chat render coalescing workloads.
• Update the README description for eca-chat-stream-flush-interval.

Benchmarks

Measured with Emacs 30.1. Baseline was origin/master at 3f2567b.
The PR branch was 12ef152.

Lower wall time is better.

op	baseline ms	PR ms	change	baseline updates	PR updates
top-level-assistant	86.74	42.83	-50.6%	0	0
subagent-one-parent	1434.59	31.10	-97.8%	1500	1
subagent-split-parents	333.28	42.12	-87.4%	1500	4
tool-prepare-all	630.13	29.52	-95.3%	600	1
tool-prepare-smart	122.30	29.60	-75.8%	120	1
mixed-stream	942.73	206.83	-78.1%	602	3
history-replay	6318.62	4782.97	-24.3%	240	240