fix: separate block markdown children

[SoonIter]

Summary

• Separate adjacent block and transparent-wrapper children with blank lines during Markdown serialization.
• Keep inline siblings, punctuation, raw Markdown text nodes, and fenced code content unchanged.
• Move sticky child-boundary coverage into src/react/render-spacing.test.tsx.
• Address review feedback by treating only existing blank-line boundaries as already separated.

Validation

• pnpm exec biome check .
• pnpm test
• pnpm build
• 2000 transparent block sibling smoke via built output: 26ms

[Copilot]

Pull request overview

This PR adjusts Markdown serialization to insert blank lines between adjacent block-level/transparent-wrapper siblings, preventing merged output like Row 1Row 2, and relocates the related regression coverage into a dedicated spacing test suite.

Changes:

• Add child-boundary spacing logic in render.ts to separate adjacent block-ish children when serializing.
• Replace inline child concatenation with a helper that conditionally inserts \n\n between certain sibling boundaries.
• Move/expand spacing-focused tests into a new render-spacing.test.ts file and remove the prior ad-hoc test from render.test.tsx.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/react/render.ts	Introduces block-boundary detection and conditional blank-line insertion between certain adjacent children during Markdown generation.
src/react/render.test.tsx	Removes the prior “two rows” adjacency test that is now covered in the dedicated spacing suite.
src/react/render-spacing.test.ts	Adds focused coverage for spacing behavior across transparent wrappers, markdown blocks, inline punctuation, and raw/fenced markdown text.

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