Code block rewrite

[RensDimmendaal]

aidialog PR #55 preserves notebook directives such as code-fold and code-summary as data-code-fold and data-code-summary attributes on Markdown code fences. This keeps the display settings available without making aidialog decide how code should appear on the page.

This PR adds a code_block callback to mdhtml’s existing rewrite() API. So Octavo can use it to read those attributes and wrap code in a collapsed section with the requested label. Other callers can use the same callback for their own code-block transformations.

[jph00]

Hey @RensDimmendaal - I refactored the machinery you're using here a bit over the last week, so I think/hope everything you need is now available, without needing this PR.

Code-block transformations are supported through rewrite(src, {'code_block': callback}), including nested blocks in lists and blockquotes. For fenced code, the callback receives info, lang, text, parsed attrs (id, classes, and pairs), plus source and character offsets. It can inspect those attributes and return replacement Markdown. Replacements preserve the orig prefixes and final newline. This provides code-folding transformations without a separate API. (Also, I've now used it to add the GFM fence-info normalization thingie you had.)

Lemme know if I've missed anything you need, or you have any q's.