Normalize Key facts and Key dates headings during import

[mikec-ai]

Closes #764.

What changed

• Normalize exact Key facts and Key dates labels to Heading 4 during import.
• Handle labels imported as headings, plain paragraphs, or bold paragraphs.
• Support labels inside a one-cell callout table or immediately before one.
• Ignore empty Word spacer paragraphs when associating a label with its table.
• Preserve substantive intervening content and leave longer labels or multi-cell tables unchanged.

Why

Word documents can represent these labels with different heading levels or as ordinary paragraph text. The existing import logic preserved those differences, so the same callout could receive an inconsistent heading level or no heading at all.

User impact

Users can import these common Word structures without manually correcting the Key facts and Key dates heading level in NOFO Builder. This normalization happens only during import, so users can still edit the content afterward.

Validation

• Added focused parser coverage for supported and intentionally excluded structures.
• Added five representative Word fixtures and exercised them through the real Mammoth import path.
• Rendered and visually checked the Word fixtures.
• 17 focused parser and Word-import tests pass.
• Full test suite passes: 1,531 tests.
• Completed an independent review and follow-up review with no remaining actionable findings.