fix(xlsx): scale Excel worksheet bar-chart value-axis label gap with font size

[developer0hye]

Summary

• Scale the Excel worksheet horizontal bar-chart value-axis label gap with the declared font size instead of a flat 7.65pt, matching the PT+EM shape already shipped for the PowerPoint sibling constant.
• Derive the new constants from one-factor native Excel probes that isolate the value-axis font size (8pt and 10pt are clean; 12pt and 14pt make native re-lay the plot itself and cannot isolate the term).
• Add real-frame regression coverage plus fresh native Excel 300 DPI evidence for the budget sheet's two percentage-label lines.

Related issue

Fixes #1622

Testing

• Pre-fix: cargo test --locked -p office2pdf an_excel_worksheet_horizontal_value_axis_uses_the_native_label_band -- --nocapture (failed: actual gap tracked the flat old constant, not a size-scaled one)
• cargo test --locked -p office2pdf an_excel_worksheet_horizontal_value_axis_scales_its_label_gap_with_size -- --nocapture (1 passed)
• cargo test --locked -p office2pdf non_worksheet_horizontal_value_axes_keep_their_existing_label_gap -- --nocapture (1 passed)
• cargo test --locked -p office2pdf a_powerpoint_horizontal_value_axis_keeps_native_label_gap_at_multiple_sizes -- --nocapture (1 passed)
• cargo test --locked -p office2pdf --lib (3089 passed)
• cargo fmt --all -- --check
• python3 scripts/probe_harness.py assets/validation/issue-1622/value-axis-label-gap-size-probe.json --backend office
• python3 scripts/probe_harness.py assets/validation/issue-1622/value-axis-label-gap-size-probe-1200.json --backend office
• python3 scripts/compare_layout.py gt_full.pdf before.pdf --page 2 --fine-shift 0.5 --noise-floor 0.5 --json (12 value-axis label instances flagged, dy -0.79/-0.80pt)
• python3 scripts/compare_layout.py gt_full.pdf after.pdf --page 2 --fine-shift 0.5 --noise-floor 0.5 --json (same 12 instances resolved; nothing else in the 547-item remaining population changed)
• python3 scripts/compare_render.py gt_full.pdf after.pdf --page 2 --dpi 300 --fine-shift 0.5 --cluster-report assets/bugfixes/issue-1622/render-clusters-page-2.json --cluster-dispositions /tmp/cluster-dispositions.json --strict-clusters (395/395 dispositioned, PASS; dispositions reused by exact bbox match from PR fix(xlsx): quantize interior horizontal bar-chart category baselines #1765's report for XLSX: horizontal bar-chart category baselines deviate from native row placement #1621 on this same fixture/page; before/after cluster sets are bit-identical — this fix's effect sits under the 5%-fuzz clustering floor at 300 DPI)

Visual impact

• No rendered PDF change
• Rendered PDF change or visual evidence added
• Reason: the horizontal bar-chart value-axis label rows in Excel worksheet bar charts now scale their gap with the declared font size instead of a flat constant.

Visual audit

• Issue: XLSX: fitted horizontal bar-chart value labels sit about 0.61pt too high #1622
• Fixture: tests/fixtures/xlsx/issue_1181_fit_to_height.xlsx (unmodified; SHA-256 2b4a2d8dceda58758593c88409875efbda05780559154c02bd13fef4f7a1c65b)
• Page(s): 2
• Renderer and DPI: Microsoft Excel for Mac native reference; pdftoppm, 300 DPI
• Evidence mode: fix
• Layout audit report: assets/bugfixes/issue-1622/layout-audit.json
• Render cluster reports: assets/bugfixes/issue-1622/render-clusters-page-2.json
• Reference exporter differences: None
• Fine-detail threshold: 0.5pt
• Layout audit page count: Pass
• Layout audit text flow: XLSX: title+header-row text on one native baseline reports as a 1-into-2 line reflow #1760
• Layout audit visible fills: Pass
• Layout audit rectangle geometry: XLSX: fitted worksheet cell fills sit one sheet point above Excel's #1632, XLSX: horizontal bar-chart value-bar fills sit ~0.544pt above native, widest bar 11pt narrow #1759
• Layout audit large shifts: Pass
• Layout audit fine shifts: XLSX: horizontal bar-chart category baselines deviate from native row placement #1621, XLSX: large left-aligned Cambria title starts 1.255pt left of native #1623, XLSX: right-aligned cells on the fitted budget sheet land 0.9pt left to 2.2pt right of Excel by number format #1631, XLSX: fitted worksheet text keeps the physical margin origin while its paint layer snaps to Excel's sheet-space origin #1719, XLSX: a 14-15pt bottom-aligned cell in a 19.5pt fixed row is centred as a tight row where Excel bottom-seats it #1721, XLSX: a form-control label row sits 2.16pt above Excel where the rest of the fitted sheet sits ~0.6-0.8pt off #1757, XLSX: repeated %INC column-header text sits 0.39pt right of native #1761
• New follow-up issues found in this audit: None
• Model vision findings: Full GT/before/after page-2 crops were inspected at 300 DPI (assets/bugfixes/issue-1622/gt.jpg/before.jpg/after.jpg), plus a 400%-zoomed crop of both charts' "0%"/"10%" value-axis labels. At normal viewing scale the defect is invisible — a -compose difference overlay of the zoomed crop is what actually shows the edge-doubling ghost around both glyphs before the fix; after the fix the same overlay is flat. The measured residual drops from -0.785pt/-0.798pt (income/expenses) to 0.0pt/0.012pt against the fresh native export. A before/after self-diff at the pixel-cluster level is bit-identical (395/395, 0 new/removed clusters) — this fix's effect sits under the 5%-fuzz clustering floor at 300 DPI, so the render-cluster disposition set is reused by exact bbox match (395/395) from PR fix(xlsx): quantize interior horizontal bar-chart category baselines #1765's report for XLSX: horizontal bar-chart category baselines deviate from native row placement #1621 on this same fixture/page (unmerged but bbox-identical, since neither PR's fix moves any pixel cluster), which individually inspected and cropped each cluster this repository has open right now: rectangle-geometry clusters map to XLSX: fitted worksheet cell fills sit one sheet point above Excel's #1632 (cell fills one sheet point high) and XLSX: horizontal bar-chart value-bar fills sit ~0.544pt above native, widest bar 11pt narrow #1759 (bar fills ~0.544pt high, one 11pt-narrow bar), and the fine-shift population maps to XLSX: horizontal bar-chart category baselines deviate from native row placement #1621 (category-label baselines), XLSX: large left-aligned Cambria title starts 1.255pt left of native #1623 (title starts 1.255pt left, confirmed by crop), XLSX: right-aligned cells on the fitted budget sheet land 0.9pt left to 2.2pt right of Excel by number format #1631 (right-aligned/number-format cells 0.9-2.2pt off, confirmed on "FEB"), XLSX: fitted worksheet text keeps the physical margin origin while its paint layer snaps to Excel's sheet-space origin #1719 and XLSX: a 14-15pt bottom-aligned cell in a 19.5pt fixed row is centred as a tight row where Excel bottom-seats it #1721 (tight-row centering, including the 14-15pt header cells "january income:"/"january expenses:" sit up to 2.16pt high), XLSX: a form-control label row sits 2.16pt above Excel where the rest of the fitted sheet sits ~0.6-0.8pt off #1757 (fitted-sheet ~0.6-0.8pt offset, confirmed on "Monthly college budget"), and XLSX: repeated %INC column-header text sits 0.39pt right of native #1761 (repeated "% INC" header text 0.39pt right). None of these move between the before and after builds in this PR.
• GT: assets/bugfixes/issue-1622/gt.jpg
• Before: assets/bugfixes/issue-1622/before.jpg
• After: assets/bugfixes/issue-1622/after.jpg
• Native: None
• Compare: assets/bugfixes/issue-1622/compare.jpg (filed with the original issue)

Visual comparison

GT	Before	After

Required inspection

• Rendered all evidence at 150 DPI or higher
• Stored progressive JPEG quality 86 assets with metadata stripped
• Used Codex/Claude vision to inspect the full GT/output pages, diff, and matched crops
• Inspected matched region crops at full resolution
• Ran compare_layout.py --audit --fine-shift PT and dispositioned every fine/large text-instance shift, rectangle geometry deviation, painted-text visibility mismatch, and visible-fill occlusion
• Ran compare_render.py --cluster-report PATH --strict-clusters and dispositioned every material 5% fuzz diff cluster by explicit ID
• Inventoried hairlines and border dash styles
• Inventoried font weight, italic, and underline emphasis

Deviation audit

Check	Result
Page count/order	Matches GT
Element presence	Matches GT
Position/size	Remaining: #1621, #1632, #1721, #1757, #1759 cover pre-existing category-label, cell-fill, tight-row, fitted-sheet, and bar-fill position; the horizontal bar-chart value-axis label rows are fixed by this PR.
Rotation/flip	No deviation observed
Fill	Remaining: #1632, #1759
Stroke/border	No deviation observed: this text-only correction changes no gridline, axis, or border stroke.
Shape outline geometry	Remaining: #1759 covers pre-existing bar-fill width; no shape geometry changes from this fix.
Text content	No deviation observed
Font family/weight/style	Matches GT for the corrected labels
Text color	Matches GT for the corrected labels
Alignment	Remaining: #1623, #1631, #1719, #1721, #1761 cover pre-existing title, cell-number-format, tight-row, and repeated-header alignment; the value-axis label vertical alignment is fixed by this PR.
Line/paragraph spacing	Remaining: #1760 covers the pre-existing 1-into-2 line reflow; single-line value-axis labels carry no paragraph spacing.
Clipping/overflow	No deviation observed

Checklist

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

Related: #1266, #1549