🐛 fix(markdown): keep block content in headings

[gaborbernat]

to_markdown let a block inside a heading break out of it. <h3><p>x</p></h3> became an empty ### followed by a paragraph x, and in <h2>a<p>b</p>c</h2> the b and c left the heading as a separate paragraph bc. 🐛

An ATX heading holds one line of inline content, so blocks inside a heading now flatten onto that line, the way link text already flattens them: the two examples give ### x and ## a b c. The end of a flattened block now reads as a space as well as its start, so link text over <div><i>a</i><i>b</i></div>x keeps a space before x where the b and x used to fuse.

markdownify 1.2.3 flattens the same way and gives ### x; html2text 2025.4.15 writes the empty ### and moves the content out, as turbohtml did. A heading nested in a heading now joins the outer heading's line, since Markdown has no nested headings.

The Markdown meaning check that runs over the #1018 fuzz consumers now compares heading content as one line too, with each block edge and <br> read as a space, since that is the only heading a reader can rebuild.

[codspeed]

Merging this PR will regress 3 benchmarks

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 2 improved benchmarks
❌ 3 regressed benchmarks
✅ 576 untouched benchmarks
⏩ 32 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	test_feature[phone-ucs4-prose]	1.3 ms	1.4 ms	-7.65%
❌	test_feature[phone-ucs2-prose]	2.1 ms	2.2 ms	-6.55%
❌	test_feature[select-relative-sibling]	42.7 µs	45 µs	-5.29%
⚡	test_feature[shadow-slot-comments]	137.9 µs	83.4 µs	+65.28%
⚡	test_feature[form-data-fieldsets]	32.5 µs	30.9 µs	+5.18%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing gaborbernat:fix/markdown-heading-blocks (3cfb39f) with main (2ffa2f0)}

Footnotes

1. 32 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩