fix(xlsx): advance bottom-legend entries by the row's measured clearance without a width floor

[developer0hye]

File submission policy

• The fixture and images derive from the existing public Gift Budget and Tracker1.xlsx misc issues #982 attachment; no new organizational file is submitted.

Summary

A worksheet axis chart's bottom legend held every entry at the 78pt LEGEND_ENTRY_W floor and skipped the visible-row trim and centre correction when the last entry fell under it, so the 6pt legend of the public Gift Budget workbook started 12.27 printed points left of native with a flat 78pt pitch where native advances 77.23 and 75.50.

Native Excel for Mac 16.112 one-factor exports (all four series renamed to one string at twelve widths at 9pt, five at 6pt and four at 18pt, plus the unchanged four-name row at 6, 9 and 18pt, each against a layout-identical re-zip control) show that an entry advances by key + gap + its own label + one clearance shared by the row, 6.617 + 0.16 x mean(label design advances), with no floor. Twenty-four of the twenty-five rows fit within 0.016pt. The earlier per-face slope table and terminal-glyph term were this share of each row's mean label width, so both are replaced by the rule. The floor and the generic GAP remain for other hosts and unmeasured faces.

On the 6pt package the three filled keys now land at 586.967 / 650.277 / 712.172 printed points against native 586.950 / 650.275 / 712.185; the 9pt row moves by under 0.006pt.

Related: #1616. Vertical seat of the 6pt row is #1748; the constrained clearance above 23pt is #1749; the line-sample box is #1617.

Related issue

Fixes #1616

Testing

• Red: an_excel_legend_entry_advances_by_its_own_label_with_no_width_floor, an_excel_legend_row_shares_one_clearance_from_its_mean_label and the compiled gift_budget_compiled_six_point_legend_keys_match_native_bounds fail on main (78pt pitch; first key 574.49 against 586.95).
• The seven-face 9/18pt gutter table and the 9pt Calibri pitch test pass unchanged under the new rule.
• cargo test --locked -p office2pdf --lib: 3085 passed.
• cargo test --locked --workspace: see the run recorded below.
• cargo clippy --locked --workspace --all-targets -- -D warnings on stable and 1.97, cargo fmt --all --check, git diff --check: passed.
• Fresh native/current PDFs of the 6pt package: compare_layout.py --audit --fine-shift 0.5 --noise-floor 0.5, compare_text_layer.py, compare_render.py --dpi 300 --strict-clusters on page 2; the text-layer census (3 fewer spaces, extraction order) is identical to the pre-fix output.
• Required delegated read-only documentation freshness audit: PASS.
• Probe specs, package and export hashes, and the measured rows: validation.

Visual impact

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

Visual audit

• Issue: XLSX: small-font bottom legends use an incorrect entry-width floor #1616
• Fixture: tests/fixtures/xlsx/issue_1603_gift_budget.xlsx (legend a:defRPr/@sz 900 to 600, the XLSX: small-font bottom legends use an incorrect entry-width floor #1616 controlled package; rebuilt in-test and by assets/validation/issue-1616/legend-size-and-label-probe.json)
• Page(s): 2
• Renderer and DPI: Microsoft Excel native reference; pdftoppm, 300 DPI
• Evidence mode: fix
• Layout audit report: assets/bugfixes/issue-1616/layout-audit.json
• Render cluster reports: assets/bugfixes/issue-1616/render-clusters-page-2.json
• Reference exporter differences: None
• Fine-detail threshold: 0.5pt
• Layout audit page count: Pass
• Layout audit text flow: XLSX: 6pt bottom legend labels seat one chart point below native #1748
• Layout audit visible fills: Pass
• Layout audit rectangle geometry: Pass
• Layout audit large shifts: Pass
• Layout audit fine shifts: Pass
• New follow-up issues found in this audit: XLSX: 6pt bottom legend labels seat one chart point below native #1748
• Model vision findings: Full GT/output pages keep the title, sidebar, chart, table and photo layout; the 5% diff is glyph-edge noise over every text run plus the sidebar B4 block (XLSX: fitted worksheet text keeps the physical margin origin while its paint layer snaps to Excel's sheet-space origin #1719) and the chart's zero-value line segments. The 2x legend crop shows all four keys and labels starting on the native x positions after the fix, with the labels sitting one chart point low (XLSX: 6pt bottom legend labels seat one chart point below native #1748), the Amount Spent label lower still under an oversized marker disc (XLSX: small-font legends retain oversized line-series markers #1617), and Excel's whole-point glyph placement widening its 6pt labels slightly. The 4x crops and a pixel column profile of the Feb-Mar and Aug-Dec segments show native clipping the 2.24pt stroke at the plot's bottom edge under the grey axis while the output paints the full stroke (XLSX: worksheet chart series are not clipped to Excel's whole-point plot rectangle, so a zero-value line paints over the category axis #1745). Every other cluster matches the XLSX: top-aligned wrapped worksheet text starts above the native baseline #1606 audit's crops by bbox.
• GT: assets/bugfixes/issue-1616/gt.jpg
• Before: assets/bugfixes/issue-1616/before.jpg
• After: assets/bugfixes/issue-1616/after.jpg
• Native: None
• Compare: None

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: two fitted pages
Element presence	Matches GT
Position/size	Remaining: #1748 6pt legend label seat; #1617 line-sample marker; #1719 sidebar B4 block; #1659 long-string drift (legend keys and labels now on native x)
Rotation/flip	Matches GT
Fill	Matches GT
Stroke/border	Remaining: #1745 unclipped zero-value line under the category axis
Shape outline geometry	Matches GT
Text content	Matches GT: baseline/current extraction identical
Font family/weight/style	Matches GT
Text color	Matches GT
Alignment	Remaining: #1748 6pt label baseline (visible legend row now centred on native)
Line/paragraph spacing	Matches GT
Clipping/overflow	Remaining: #1745

Checklist

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

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