docs: add PR 186 visual evidence

[developer0hye]

Summary

Adds the missing visual regression evidence for #186 under assets/bugfixes/issue-186/.

Ground truth (Excel)	Before #186	After #186

Provenance

All three images use tests/fixtures/xlsx/pr_186_contributor_acceptance.xlsx:

• GT: page 1 of Microsoft Excel native PDF export
• Before: page 1 rendered from the fixture-only branch before fix(xlsx): cell alignment, column width, borders, page setup #186
• After: page 1 rendered from fix(xlsx): cell alignment, column width, borders, page setup #186 with the completed umya-spreadsheet inline-string fix

The after image now matches the native 612 x 396 pt Statement landscape page, right-aligns numeric 1250, preserves text 0050, and renders the double top border as a solid heavy line.

Verification

• PDF text extraction confirms 0050 is preserved in the after output
• identify confirms GT and after are both 1224 x 792 px at 144 DPI
• generated assets were visually inspected

Signed-off-by: Yonghye Kwon developer.0hye@gmail.com