fix: separate block markdown children (#7)

## 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

Author: SoonIter

src/react/render-spacing.test.tsx

@@ -0,0 +1,188 @@
+import { describe, expect, it } from 'vitest';
+import { renderToMarkdownString } from './render';
+
+describe('renderToMarkdownString - child boundary spacing', () => {
+  it('separates adjacent transparent block wrappers', async () => {
+    expect(
+      await renderToMarkdownString(
+        <>
+          <div>Row 1</div>
+          <div>Row 2</div>
+        </>,
+      ),
+    ).toMatchInlineSnapshot(`
+      "Row 1
+
+      Row 2"
+    `);
+  });
+
+  it('separates nested transparent block wrappers', async () => {
+    expect(
+      await renderToMarkdownString(
+        <main>
+          <section>
+            <div>Intro</div>
+          </section>
+          <article>
+            <div>Details</div>
+          </article>
+        </main>,
+      ),
+    ).toMatchInlineSnapshot(`
+      "Intro
+
+      Details"
+    `);
+  });
+
+  it('separates text from adjacent block wrappers in flow containers', async () => {
+    expect(
+      await renderToMarkdownString(
+        <>
+          Lead
+          <section>Section</section>
+          <aside>Aside</aside>
+          Tail
+        </>,
+      ),
+    ).toMatchInlineSnapshot(`
+      "Lead
+
+      Section
+
+      Aside
+
+      Tail"
+    `);
+  });
+
+  it('separates blocks even when the previous text ends with a space', async () => {
+    expect(
+      await renderToMarkdownString(
+        <>
+          {'Lead '}
+          <section>Section</section>
+        </>,
+      ),
+    ).toMatchInlineSnapshot(`
+      "Lead 
+
+      Section"
+    `);
+  });
+
+  it('completes block separation when the boundary already has one newline', async () => {
+    expect(
+      await renderToMarkdownString(
+        <>
+          {'Lead\n'}
+          <section>Section</section>
+        </>,
+      ),
+    ).toMatchInlineSnapshot(`
+      "Lead
+
+      Section"
+    `);
+  });
+
+  it('does not over-separate blocks that already start with one newline', async () => {
+    expect(
+      await renderToMarkdownString(
+        <>
+          {'Lead'}
+          <pre data-lang="ts">
+            <code>{'const value = 1;\n'}</code>
+          </pre>
+        </>,
+      ),
+    ).toMatchInlineSnapshot(`
+      "Lead
+
+      \`\`\`ts
+      const value = 1;
+
+      \`\`\`
+      "
+    `);
+  });
+
+  it('does not add duplicate spacing around markdown blocks that already delimit themselves', async () => {
+    expect(
+      await renderToMarkdownString(
+        <div>
+          <h2>Title</h2>
+          <p>Paragraph</p>
+          <div>Footer</div>
+        </div>,
+      ),
+    ).toMatchInlineSnapshot(`
+      "## Title
+
+      Paragraph
+
+      Footer"
+    `);
+  });
+
+  it('keeps inline siblings and punctuation joined exactly', async () => {
+    expect(
+      await renderToMarkdownString(
+        <p>
+          <strong>foo</strong>
+          <span>bar</span>,<a href="/docs">docs</a>.
+        </p>,
+      ),
+    ).toMatchInlineSnapshot(`
+      "**foo**bar,[docs](/docs).
+
+      "
+    `);
+  });
+
+  it('does not insert spaces into comma-separated inline React children', async () => {
+    expect(
+      await renderToMarkdownString(<p>ref-value,memo-value,provided</p>),
+    ).toMatchInlineSnapshot(`
+      "ref-value,memo-value,provided
+
+      "
+    `);
+  });
+
+  it('keeps raw markdown text nodes untouched', async () => {
+    expect(
+      await renderToMarkdownString(
+        <>
+          {'# Code Example\\n'}
+          {'\n'}
+          {'```tsx\nconsole.log("Hello, world!");\n```\n'}
+        </>,
+      ),
+    ).toMatchInlineSnapshot(`
+      "# Code Example\\n
+      \`\`\`tsx
+      console.log("Hello, world!");
+      \`\`\`
+      "
+    `);
+  });
+
+  it('keeps fenced code block content untouched', async () => {
+    expect(
+      await renderToMarkdownString(
+        <pre data-lang="ts">
+          <code>{'const values = ["a", "b"];\n'}</code>
+        </pre>,
+      ),
+    ).toMatchInlineSnapshot(`
+      "
+      \`\`\`ts
+      const values = ["a", "b"];
+
+      \`\`\`
+      "
+    `);
+  });
+});

src/react/render.test.tsx

@@ -264,21 +264,6 @@ describe('renderToMarkdownString - styles', () => {
       "
     `);
   });
-
-  it('renders two row correctly', async () => {
-    const Comp1 = () => {
-      return (
-        <>
-          <div>Row 1</div>
-          <div>Row 2</div>
-        </>
-      );
-    };
-
-    expect(await renderToMarkdownString(<Comp1 />)).toMatchInlineSnapshot(
-      `"Row 1Row 2"`,
-    );
-  });
 });
 
 describe('renderToMarkdownString - effects never execute (SSR behavior)', () => {