QuickBook Parse/Serialize Helpers (`utils/quickbook.py`)

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Problem

There is no standard po4a-style external converter for QuickBook. To support translation in Weblate, the project needs an in-process parser that extracts translatable segments into a translate-toolkit store, plus reconstruction that applies translations while preserving non-translatable markup and layout.

Acceptance Criteria

• Parser extracts agreed translatable constructs (paragraphs, lists, headings, sections, admonitions, block quotes, selected table content, variable lists, etc.) and skips or copies verbatim the documented non-translatable constructs (code blocks, includes, templates, comments, etc.).
• Inline QuickBook markup inside translatable text remains in the unit source so translators can preserve it.
• Nested sections are handled with a sane recursion strategy (including a depth limit) so nested translatable blocks are not dropped.
• QuickBookTranslator (or equivalent) can rebuild .qbk from template bytes + PO/store lookup, analogous in spirit to translate.convert.po2asciidoc.AsciiDocTranslator.

Implementation Notes

• Implement on top of translate.storage abstractions already used elsewhere in the stack.
• Prefer explicit keyword/skip lists and regex boundaries that are easy to extend; avoid silent partial parses.

References

• src/boost_weblate/utils/quickbook.py
• translate.convert.po2asciidoc (pattern reference for PO → structured text)
• Boost QuickBook documentation