fix: double-clicking a code block in preview keeps your place

[productengineered]

In preview, double-clicking inside a code block opened the editor at the top of the document instead of at the click. Double-clicking in the margin, or a gap between blocks, did the same.

Smallest repro: any document long enough to scroll, with a fence below the fold:

(a few screens of paragraphs)

```
Create two empty folders in this project: drafts and published.
```

Open it (it lands in preview), scroll to the fence, and double-click a word in it. The editor opens at line 1.

What

Cause: Shiki replaces each <pre> it highlights with a new node, and the data-pos stamp from rehypeSourcePositions is lost along with the old one. The dblclick handler then finds no stamped ancestor and calls onEdit(0).

• src/shared/markdown.ts (only when sourcePositions is on): record the <pre> stamps before the highlighter runs and put them back after it, in document order. Shiki swaps one <pre> for one <pre> in place, so the order holds. The stamps are stored on the vfile rather than in plugin state, because the processor is shared and renders can overlap.
• src/renderer/ui/preview.ts:
  • A double-click that lands on no stamped element uses the nearest block, the same lookup visibleAnchor uses. The page holds still and the caret goes to that block.
  • New sourceOffsetNear handles a click on whitespace, such as the gap between words or a code line's indentation. It maps the neighbouring word, then steps across the gap when the source has the same characters there (in a code block it always does). This puts the caret on the clicked character rather than at the start of the block.

Tests

• Layer: unit tests for the pipeline and the pure function, plus an end-to-end case for preview.ts.
• Case / test added or extended:
  • markdown.test.ts: code blocks keep their stamps when sourcePositions is on. The cases are a plain fence, a lazily loaded language, an unknown language, and a fence nested in a list. It failed before the fix ([]).
  • Golden files: no diff. The change only runs when sourcePositions is on, and exports never turn it on.
  • preview.test.ts: sourceOffsetNear covers clicks in a word, in a gap between words, in indentation, across a gap whose source differs, and on a line with no word at all.
  • scripts/e2e/cases/preview-dblclick.mjs: double-clicks a word in a code block deep in a long document, then the margin beside a paragraph. It checks that the editor isn't at the top, that the caret is on the clicked character or paragraph, and that the line holds its height on screen. On main it fails with the editor's scrollTop at 0.
• pnpm typecheck && pnpm test pass (376 tests). pnpm e2e passes (7/7), and pnpm test:coverage stays above the floors.