Treat picture as a block-level HTML element

[00200200]

Description

A line-started <picture> with an HTML comment inside it was split across paragraphs, because picture was treated as inline HTML while comments are raw blocks. That produced invalid markup such as <p><picture>…<!-- … --></p><p><source>…</picture></p>.

Treat picture as a block-level HTML element, matching video and other media tags whose contents Markdown should not rewrite. A <picture> that appears mid-paragraph remains inline.

Fixes #1643.

AI Assistance Disclosure

• No AI tools were used in preparing this PR.
• If AI tools were used, I have disclosed which ones, and fully reviewed and verified their output.

AI tools (Cursor) were used to locate the issue, draft the change, and write tests. I reviewed the parser behavior, confirmed the test fails on master and passes after the change, and ran the unit suite.

Checklist

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• The code follows the Code Style Guide.
• The commit message follows the Commit Message Style Guide.
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• I have added or updated relevant tests.
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[facelessuser]

<picture> is an inline element, though. That is how it is treated by default in browsers.

[waylan]

As the HTML spec clearly identifies <picture> as an inline element, we will not be making this change.