fix(editor): keep a blockquote's blank line and its nesting on the way back

[h4yfans]

Summary

A blockquote's own structure never survived a save. > A\n>\n> B came back as > A\n> B, and a nested > > [!warning] callout came back unnested. On an Obsidian vault that is a rewrite of a file Memry never wrote (#1881, and the byte-compat guarantee #1846 exists to protect). #1875 deliberately declined these shapes from the callout claim, which left them on the generic quote path where the damage happens.

Serialization was never the culprit. BlockNote parses a whole blockquote into ONE block of inline content, so the blank > separator and the > > level are gone before serialization runs. Measured: > [!info]\n> One\n>\n> Two parses to text: "[!info]\nOne\nTwo\n". BlockNote's quote block cannot hold the structure either, since its children serialize outside the quote (> A\n\nB) and a \n\n in content becomes backslash hard breaks. So the fix is at the parse seam, not the serializer.

A blockquote run carrying a blank > separator or a > > level is now claimed whole. One > level comes off, the inside parses as ordinary markdown, and the first block becomes the quote's content with the rest as its children. serializeQuote writes that list back inside the blockquote, one > per line and a bare > per gap.

The claim is proven, not pattern-matched, the same rule resolveCalloutRun already applies: re-serializing the parse must reproduce the run byte-for-byte, or the run is left exactly where it was. A run with neither shape is already flat and never enters this path, so no quote that round-trips today changes.

Reviewer notes:

• No new node type. quote and paragraph only, so a doc written by this build still opens on an older install rather than hitting y-prosemirror's delete-unknown-node repair.
• Both pipelines share the seam. splitMarkdownByCallouts is now splitMarkdownByBlockquoteRuns and emits quote runs alongside callout runs; the main process claims in its existing line loop.
• Blockquote blank lines collapse on write-back, merging quoted paragraphs #1881 stays open. A lazily continued > Outer\n> > Inner (no blank line between the levels) only comes back through a separator the author never wrote, so it declines on the byte proof and keeps today's flat behavior. It is pinned as a new pending corpus case rather than left undocumented.
• First commit adds the failing shapes to the corpus as pending cases; the second lands the fix and removes the flags. Each commit is green on its own.

Release note

Quotes with more than one paragraph, and quotes nested inside other quotes, now survive a save instead of being flattened. Notes written in Obsidian with these shapes are written back byte for byte.

Test plan

• vitest --project main src/main/sync/blocknote-converter.roundtrip.test.ts — 59 passed, 3 expected fail (the remaining pending cases: Blockquote blank lines collapse on write-back, merging quoted paragraphs #1881 lazy continuation, Unterminated toggle drops its open and summary lines on write-back #1883, Blank-line gaps adjacent to a toggle collapse on write-back #1877)
• vitest --project renderer src/renderer/src/components/note/content-area/roundtrip-conformance.test.ts — 51 passed, 5 expected fail
• New golden fixture roundtrip-quotes.md round-trips byte-identical through the converter
• New unit tests for readStructuredQuoteRun / serializeQuoteBlock covering the decline rules (flat run, >text line, non-quote line)
• Full desktop suites: 10206 passed | 4 expected fail (main + shared), 8524 passed | 5 expected fail (renderer)
• tsc --noEmit on tsconfig.node.json and tsconfig.web.json, eslint and prettier clean on touched files
• pnpm docs:impact --base <base> --strict covered, pnpm docs:build passes
• Rebased onto current main and re-run against its new raw-html-roundtrip.probe.test.ts suite

[github-actions]

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