fix(desktop): reduce streaming render jitter (webview coalescing + height-cache hygiene)

[Lxiny-zy]

Problem

Streaming rendering jitter on fast model streams. Two compounding sources:

1. Per-chunk IPC floods the webview channel. The agent emits one event per model chunk (internal/agent/agent.go emits Text/Reasoning per chunk); tabEventSink.Emit forwards every event to Wails runtime.EventsEmit one by one. When the single webview event channel backs up, EventsEmit blocks, backlog latency spikes, and the frontend's per-frame batch drains a stale backlog in one jump instead of a smooth stream.
2. Mid-stream heights poison the measurement cache. Streaming rows are measured every frame (tail growth, live code highlighting); those transient heights were written into transcriptHeightCache, so a later re-mount of the same row key (history replay, tab switch) started from a mid-stream height and settled with a second viewport jump.

Fix

1. webviewStreamCoalescer (new, desktop/tab_stream_coalescer.go): merges consecutive same-kind text/reasoning deltas for the same tab+submission into one wire event per 16ms window (one 60Hz frame — the frontend already coalesces a second time per frame, so the merged rate never exceeds the repaint rate). Event order and the reasoning→text boundary are preserved (a kind or submission switch flushes immediately). The merge sits only on the webview exit: bot sinks, project-tree metadata, telemetry and the display buffer keep receiving every event unchanged.
2. Streaming rows never write the height cache. createTranscriptMeasureElement accepts a skipCacheWriteWhen predicate; useTranscriptRowMeasurements matches answer rows whose item.streaming is still true. Settled rows keep writing their final measured height.

Tests

Go (tab_stream_coalescer_test.go, all pass; full go test ./desktop/... green):

• window merge of consecutive text deltas
• reasoning→text boundary flushes separately
• different submissions never merge
• non-stream events (e.g. turn_done) flush the pending delta immediately
• window timer flushes a lone delta without further pushes
• sink-level: bot sinks still receive every event unchanged while the webview gets the merged stream

Frontend (transcript-height-cache.test.ts, 5 assertions; tsc --noEmit and eslint clean):

• predicate matching for streaming/settled/user/missing rows
• integration check that only settled rows reach the cache through the measure element

Cache-impact: none - frontend rendering path + Wails event bridge only; no prompt or tool schema bytes change
Cache-guard: none
System-prompt-review: none - no prompt, config schema, memory, output style, or skill behavior changes
Documentation-impact: none - internal rendering/IPC performance fix; no CLI, docs or config surface changed

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[Lxiny-zy]

Superseded alongside #8291 by #8264's unified transcript scroll/rendering rework; closing to keep the PR list clean. The Go-side webview stream coalescer (per-chunk IPC merging) remains available here if it is ever wanted.