fix: separate block markdown children

[SoonIter]

Summary

• Backport block/transparent-wrapper child boundary spacing to the v18 branch.
• Keep inline siblings, punctuation, raw Markdown text nodes, and fenced code content unchanged.
• Add dedicated src/react/render-spacing.test.tsx coverage for sticky child-boundary cases.
• Treat only existing blank-line boundaries as already separated.

Validation

• pnpm install
• pnpm exec biome check .
• pnpm test
• pnpm build
• 2000 transparent block sibling smoke via built output: 19ms

[Copilot]

Pull request overview

This PR adjusts the React-to-Markdown serializer to insert appropriate blank-line spacing at child boundaries for top-level / “transparent” block containers (e.g. div, section, main), bringing block sibling output in line with expected Markdown paragraph separation while preserving inline concatenation and fenced-code behavior.

Changes:

• Introduces block-boundary detection and a separator algorithm to insert \n / \n\n between adjacent block-ish children only when needed.
• Refactors child serialization into a dedicated renderChildren() path and uses it throughout toMarkdown().
• Moves/expands coverage into a new render-spacing.test.tsx suite for “sticky” spacing cases and removes the superseded spacing test from render.test.tsx.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/react/render.ts	Adds flow-container/block-boundary separation logic and routes child rendering through renderChildren() to enforce consistent spacing rules.
src/react/render.test.tsx	Removes an outdated two-row spacing test now covered more comprehensively elsewhere.
src/react/render-spacing.test.tsx	Adds focused tests covering transparent wrapper boundaries, newline completion, inline punctuation stability, and fenced-code/raw-text invariants.

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