fix(asciidoc): keep a titled paragraph and the text after it

[r0h1tb]

Problem

A block title in front of a paragraph drops the paragraph and all the text after it:

src = b"Intro paragraph.\n\n.Block title\nTitled paragraph.\n\nAfter paragraph.\n"
doc.export_to_markdown()  # 'Intro paragraph.'

After .Block title, the "allow multiline captions" branch appends every following non-empty line to caption_data. That buffer is only emitted when a table, picture, list or literal block comes next, so everything up to that point, or to the end of the document, is lost without an error. A block title with nothing after it is lost the same way.

Fix

A block title is a single line, so the line after it starts the paragraph it titles. The block title is now emitted as a bold paragraph just before it, which is what _parse's docstring already describes for targets without a caption slot, and what the list branch does. The pending title is also flushed at the end of the document. The list branch now uses the same helper; its behaviour is unchanged.

Tests

Two new tests in tests/test_backend_asciidoc.py: a titled paragraph followed by more text (exact labels, texts and order, bold title), and a block title at the end of the document. Both fail on main. The asciidoc module goes from 17 passed to 19, with the example groundtruth unchanged. ruff is clean and ty reports no new diagnostics beyond the file's existing _backend.convert() warnings.

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

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