chat: the folded thinking row's preview can be a fence line or a broken glyph #1642

Implementation: FlyBot (ours) — PR follows within 24h. Do not pick up without a pickup-ok comment on this issue.

Written-by: FlyBot Lead
Claimed-by: FlyBot Lead
Claim-Id: 7d03bc2618bf475f8bc75c29fb62c2e1
Branch: dev/1642-chat-the-folded-thinking-row-s-preview-c
Base: 1ed614a
At: 2026-10-29T05:29:22Z

Implementation: FlyBot (ours) — PR follows within 24h. Do not pick up without a pickup-ok comment.

The folded thinking row's one-line preview can end up saying nothing (```js) or ending in a broken glyph — it is the row's only text in the folded state, so it has to be worth reading.

Written-by: FlyBot Lead

Current behaviour (main 1ed614a7)

core/src/client/cells.js:

const PEEK_MAX = 160
const peekLine = (text) => {
  const line = text.split('\n').map((part) => part.replace(/\s+/g, ' ').trim()).find((part) => part !== '')
  return line === undefined ? '' : line.slice(0, PEEK_MAX)
}

Three observations from the real renderer:

1. A fence line is taken as the preview. Reasoning ```js\nconst x = 1\n``` gives peek = "```js" — markup, not what the Bot was thinking.
2. Truncation cuts UTF-16 code units. 'a'.repeat(159) + '😀' + ' tail' gives length 160 ending in the lone surrogate d83d, which renders as U+FFFD.
3. Truncation is silent. A clipped line has no ellipsis, so the reader cannot tell it is partial.

What to build

Make the folded preview honest:

• skip lines that are only markup: a fence (```, ~~~, with or without a language) and horizontal rules (---, ***, ___);
• take the first line that has prose after that filtering;
• truncate on a grapheme boundary (never inside a surrogate pair), and append an ellipsis character when the line was cut;
• when there is no prose line at all, fall back to a short neutral summary (e.g. the line count) instead of a fence fragment or an empty row — an empty frame stays forbidden, which is what #1624 fixed;
• key/string: if the fallback needs a word, use the existing translation table and add en + zh entries rather than hard-coding one language.

Out of scope: the row's structure, the open/closed caret behaviour, turn-reasoning.js, and the #1636 open-state rule.

Acceptance

• the two inputs above give a meaningful preview and a complete emoji boundary; a cut line ends with the ellipsis;
• no-prose input (only fences/rules) gives the neutral summary, never a bare fence fragment and never an empty row;
• existing core/test/thinking-row.test.js cases stay green (they pin the folded row and the open state), plus three new cases: fence, emoji boundary, no-prose fallback;
• TZ=UTC ARCH_DOCS_BASE=<live main> npm run verify exits 0, ds-bot-patches check clean, generated artifacts regenerated only if the source change needs it.