fix: key section folds by unique heading id instead of heading text

[PathGao]

Bug

Fold state (collapsedHeaders) is keyed by h.id || textContent. comrak's header_ids extension puts the deduplicated id (title, title-1, …) on an empty inner <a class="anchor">, not on the heading element itself — so h.id is always empty and every consumer silently falls back to heading text. Two consequences:

1. Duplicate titles share one fold state. Collapse one "## Notes" section, and after the next re-render every section titled "Notes" is collapsed.
2. The ToC fold button and the preview disagree on keys. The ToC keys by anchor id (notes), the preview restore keys by text (Notes) — they only coincide when a title equals its own slug (single lowercase word), so folding from the ToC usually doesn't collapse the section body at all.

Fix

Promote the anchor's id onto the heading element (and remove it from the anchor, keeping document ids unique) in processMarkdownHtml's heading pass. All four fold consumers — preview chevron, render restore, ToC fold button, and toggleFold's [id=…].foldable-header lookup — already prefer h.id and now agree on one unique key, with no change to their logic. In-document #anchor links still resolve (the id moves from the empty inline anchor to the heading that contains it, same scroll target).

Verification

• npm run check: 0 errors / 0 warnings
• All script tests pass (59/59), including a new scripts/foldKeys.test.ts covering the id promotion and the key precedence at each consumer

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