Restore guidance for mistagged Word headings

[mikec-ai]

Closes #776.

What changed

• Restores specific, actionable guidance when paragraph-length content is accidentally styled as a Word heading.
• Identifies the affected heading content without exposing unrelated validation details.
• Applies the same handling across Builder, Compare, and Composer import paths.
• Makes Builder re-import atomic so a failed import cannot remove or partially replace the existing NOFO.
• Keeps unexpected validation failures on the existing generic, safe error pages.

Why

The standardized import error work in #748 caused a known mistagged-heading error to fall through to generic IMPORT-CREATE-INVALID guidance. Editors could no longer tell which paragraph had the incorrect Word style or how to repair the document.

Validation

• Added a real .docx fixture containing an ordinary paragraph accidentally styled as a heading.
• Exercised new Builder import, Builder re-import, new Compare import, Compare import into an existing document, and Composer import.
• Verified each route returns IMPORT-HEADING-TOO-LONG with the affected content rather than its generic fallback.
• Verified a failed Builder re-import leaves the original sections and subsections unchanged.
• Rendered and visually inspected the Word fixture.
• Full test suite: 1,581 tests passing.

[benbrasso]

Approved to merge