feat(usfm): preserve section headings through import and export

[henrique221]

USFM import could append a section heading to the previous verse, and chapter content could drop stored headings. I now keep supported titles in the existing markers.headings field and preserve their order through import, editing, chapter content and export. Export also adds a body paragraph after a heading when needed, keeping scripture text outside the heading.

This is a draft stacked on #305 because the import path is not on main yet. Please retarget this PR to main before merging #305, since merged branches are deleted automatically. No new migration or route is required. Refs #288; the web fix is eten-tech-foundation/fluent-web#432.

Validation: all 607 tests passed, plus lint, formatting, typecheck, build and the docs check. An isolated PostgreSQL 16 smoke passed for immediate and delayed imports, saved edits, chapter content, export, empty titled verses, retries without overwrites and compatibility with legacy semantic-division records. Lint has three existing file-length warnings. The full pre-merge workflow does not run for a draft targeting the import branch.

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