fix(xlsx): carry the workbook Normal font onto unstyled cells

[developer0hye]

Summary

• carry the workbook Normal font (styles.xml font 0) onto cells whose own style names no font or size
• stop discarding a cell font because it is Calibri, or a size because it is 11.0
• return NormalFont from extract_normal_font and derive the column print metric next to it

Related issue

Fixes #462

Details

A cell with no s attribute uses cellXfs[0], whose font is the workbook Normal font. umya reports no font at all for such a cell, so the style path returned TextStyle::default() and the renderer picked its own family and size.

Two special cases compounded it: the family was discarded when it was "Calibri" and the size when it was 11.0, on the grounds that they were "the Excel default". Both left the most common workbooks to the renderer's serif default rather than to Calibri, which font substitution resolves perfectly well. A rich-text run naming Calibri was ignored for the same reason.

pdffonts on the regression fixture, whose Normal font is Malgun Gothic 12:

	before	after
embedded faces	LibertinusSerif-Regular, Calibri	MalgunGothic, Calibri

The Libertinus fallback is gone: nothing on the page falls back to the renderer default any more.

Testing

• cargo fmt --all -- --check
• cargo test --workspace
• cargo clippy --workspace --all-targets --all-features
• pdffonts before/after comparison on tests/fixtures/xlsx/theme_color_drawing.xlsx
• pdftoppm -r 150 rendering, matched full-resolution title crops, and magick compare -metric AE -fuzz 5%

Visual impact

• No rendered PDF change
• Rendered PDF change or visual evidence added
• Reason: unstyled cells now render in the workbook Normal font and size.

Visual audit

• Issue: XLSX: unstyled cells lose the workbook Normal font #462
• Fixture: tests/fixtures/xlsx/theme_color_drawing.xlsx
• Page(s): page 1 of the Excel GT and of the office2pdf output
• Renderer and DPI: pdftoppm, 150 DPI
• Evidence mode: fix
• New follow-up issues found in this audit: None
• GT: assets/bugfixes/issue-462/gt.jpg
• Before: assets/bugfixes/issue-462/before.jpg
• After: assets/bugfixes/issue-462/after.jpg

The GT is the native Microsoft Excel export already captured for this fixture in #434, reused unchanged: same workbook, same page, same renderer, same 150 DPI.

Visual comparison

GT	Before	After

Required inspection

• Rendered all evidence at 150 DPI or higher
• Stored progressive JPEG quality 86 assets with metadata stripped
• Inspected matched region crops at full resolution
• Ran the 5% fuzz pixel-difference sweep
• Inventoried hairlines and border dash styles
• Inventoried font weight, italic, and underline emphasis

The 5% fuzz sweep against the size-normalized GT moves from 290,500 to 290,889 differing pixels: the drawing geometry defects tracked in #459 and #460 displace all three shapes and dominate the count, and the now-wider sans-serif title covers slightly more pixels than the narrow serif it replaced. The matched title crops at full resolution show the change directly — the GT and the after image both set the A1 title in the workbook's 12pt sans-serif Normal font, the before image in an 11pt serif. The fixture contains no hairlines, borders, or dashed strokes, and every run is regular weight with no italic or underline.

Deviation audit

Check	Result
Page count/order	Remaining: #459 - Excel emits one page; office2pdf adds a blank second page
Element presence	Matches GT - the title and all three drawing labels are present
Position/size	Remaining: #459 tracks same-row vertical stacking; #460 tracks undersized anchor bounds and cumulative horizontal drift
Rotation/flip	No deviation observed - the fixture contains no rotated or flipped elements
Fill	Matches GT - all three RGB values match Excel exactly after #430
Stroke/border	No deviation observed - the shapes intentionally have no outline or hairline
Text content	Matches GT - all four strings are preserved
Font family/weight/style	Fixed - the A1 title now renders the workbook's 12pt Malgun Gothic Normal font, as in Excel; the drawing labels keep the Calibri resolved in #461
Text color	Matches GT - all text remains black
Alignment	Matches GT - the title is left-aligned and the drawing labels stay centered
Line/paragraph spacing	No deviation observed - every text region is one regular single-line paragraph
Clipping/overflow	Remaining: #459 - flow-height reservation spills onto a blank second page; no page-1 glyph clipping was found

Checklist

• Commits include a Signed-off-by line
• PR scope contains one root cause
• Remaining visual deviations each reference an open issue