fix: accept explicit no-underline stylesheet fonts

[baseballyama]

Summary

Allow stylesheet fonts containing <u val="none"/> to load and round-trip instead of throwing OpenXmlSchemaError. Keep the explicit value in cell and differential fonts, and render it as no underline in fontToCss.

Motivation

Closes #206.

FontSchema uses UNDERLINE_STYLES to validate XML, but that list omitted none, a valid ST_UnderlineValues member in the vendored SpreadsheetML XSD and Microsoft's reference. Both workbook loaders therefore rejected the report described in the issue. A synthetic XLSX reproduces the same exception without needing the reporter's private data.

Changes

• Accept and preserve none in the existing UnderlineStyle model and font schema. Keeping it distinct from an absent field also preserves explicit overrides in differential styles.
• Emit text-decoration: none for explicit no-underline fonts, while retaining strike-through when set.
• Document the value on the existing font APIs and add a patch changeset.

Testing

• pnpm vitest run tests/styles/issue-206.test.ts: 13 cases cover all valid underline values, bare/absent elements, invalid values, CSS, regular and streaming loading, editing, and save/reload. Four cases failed with the reported exception before the fix.
• The synthetic input and saved XLSX both pass OPC, XSD and semantic conformance validation.
• Full suite: 3,196 tests passed; lint, typecheck, knip, build, size, check:attw, and check:consumer passed.
• PERF_GATE=1 pnpm test:perf passed.

Breaking changes

None. Previously accepted values and the bare <u/> default retain their behavior.

Checklist

• I have read CLAUDE.md and followed the project's conventions.
• I have added or updated tests for the change.
• I have added or updated documentation where user-visible behavior changed.
• If this is a user-visible change, I have run pnpm changeset and committed
  the result.
• If this is a breaking change, I have flagged it above and the changeset is
  marked accordingly.
• I have re-read my own diff and removed dead code, debug prints, and stale
  comments.
• If I used an LLM to draft this PR, I have verified each change myself, this
  PR represents real work that warrants a maintainer's review, and I am
  willing to defend each line in review.

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