* fix(app): cap oversized assistant messages
A single assistant message can expand into enough rendered Markdown nodes to crash the renderer. Limit the Markdown input while preserving the complete timeline value and report its full byte size.
* fix(app): bound oversized stream processing
Cap the render input before paced reveal and defer full-message byte counting until completion. Reuse the existing grapheme boundary policy so capped text never ends in a partial glyph.
* fix(app): preserve capped text without segmentation
Hermes lacks Intl.Segmenter, while the shared reveal boundary intentionally returns zero without it. Keep that contract and fall back locally to a surrogate-safe 32K prefix.
