fix(xlsx): draw icon-set arrows as Excel's filled shapes

[developer0hye]

Summary

• Draw XLSX icon-set arrows as Excel's filled shapes — a shaft with a triangular head, in the band color with a darker outline — instead of triangle characters roughly a third their size.
• Diagonal arrows are the same shape rotated 45 degrees.
• Icon sets that are not arrows (traffic lights, flags, symbols) keep their character rendering.
• The arrow codepoints moved into the IR so the parser and renderer share one vocabulary for TableCell::icon_text.

Related issue

Fixes #377

Testing

• cargo test --workspace (1823 tests, all pass)
• cargo clippy --workspace --all-targets (clean)
• cargo fmt --check
• pdftoppm -r 150 on 10_kpi_tracker_en page 1 for all three PDFs
• Matched-region crops of the Achievement column at 280% scale
• magick compare -metric AE -fuzz 5% on the size-normalized page

Visual impact

• No rendered PDF change
• Rendered PDF change or visual evidence added
• Reason:

Visual audit

• Issue: XLSX: icon set glyphs render thin text arrows instead of Excel's solid icons #377
• Fixture: tests/golden_mocks/business/sources/xlsx/10_kpi_tracker_en.xlsx
• Page(s): 1
• Renderer and DPI: pdftoppm, 150 DPI
• Evidence mode: fix
• New follow-up issues found in this audit: None
• GT: assets/bugfixes/issue-377/gt.jpg
• Before: assets/bugfixes/issue-377/before.jpg
• After: assets/bugfixes/issue-377/after.jpg
• Compare: assets/bugfixes/issue-377/compare.jpg

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

Deviation audit

Check	Result
Page count/order	Matches GT: one page
Element presence	Matches GT: every icon, value, and header cell is present
Position/size	Fixed: arrows now fill the row the way Excel's icons do instead of rendering at a third the size
Rotation/flip	Matches GT: up, right, and down bands point the way Excel draws them
Fill	Fixed: arrows are filled in the band color with a darker outline
Stroke/border	Matches GT: the cell grid rules are present at matching positions
Text content	Matches GT
Font family/weight/style	Matches GT
Text color	Matches GT: the red 84% value keeps its conditional font color
Alignment	Matches GT: icons stay at the cell's left edge with the value on the same line
Line/paragraph spacing	Matches GT: the row tracks are unchanged
Clipping/overflow	No deviation observed

Checklist

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