fix(xlsx): resolve drawing-only sheet anchors with the Normal-font metric

[developer0hye]

Summary

Sheets with an empty <sheetData/> but drawings/text boxes/charts resolved their anchors through empty_sheet_context(), which hardcoded max_digit_width_px: 7.0 and dropped the workbook Normal font — laying every drawing-only sheet out on 44.2575pt columns where the converter's own Calibri-11 metric says 50.58pt. Pictures were 12.5% narrow and 8% aspect-distorted.

The digit-width derivation is now a single shared helper resolve_max_digit_width_px (Normal-font metric, falling back to the dominant-cell-font path that degrades to the legacy 7px on an empty sheet with unreadable styles). prepare_sheet_context uses the same helper — a behavior-identical extraction — so the metric cannot drift between populated and drawing-only sheets. Both empty_sheet_context call sites (parse and streaming parse) pass the Normal font through.

Related issue

Fixes #620

Testing

• cargo test --workspace — green (new red-first tests: an in-memory drawing-only workbook whose spliced twoCellAnchor picture must resolve x/width at the 50.58pt metric with the 44.2575pt legacy negative-asserted; a triangulation test pinning Calibri-11→8px, Calibri-8→6px, None→7px so a hardcoded 8.0 cannot pass)
• cargo +1.97 clippy --workspace --all-targets — clean; cargo fmt --check — clean
• Release-build measurement vs a fresh local Excel-for-Mac GT export: column width solves to exactly 50.58pt from three independent anchors across WithDrawing/WithTextBox (was 44.2575); tomcat.png x error −69.5pt → −19.0pt; clock.jpg width error −16.5% → −4.6%
• Populated-sheet regression: 05_project_schedule_en trace byte-identical to main

Visual impact

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

Visual audit

• Issue: XLSX: drawing-only sheets lay out columns with a hardcoded 7px digit width, so pictures are 8% distorted and a 2-page sheet collapses to 1 #620
• Fixture: tests/fixtures/xlsx/poi/WithDrawing.xlsx
• Page(s): 1
• Renderer and DPI: pdftoppm, 150 DPI
• Evidence mode: fix
• New follow-up issues found in this audit: XLSX: drawing extents never map to horizontal page breaks, so a drawing-only sheet Excel prints on 2 pages collapses to 1 #713, XLSX: drawing-only sheets ignore <cols> widths and price every column at the 8.43-char default, so anchored pictures shrink 58% and land 139pt off #714, XLSX: drawing anchors honour the declared defaultRowHeight where Excel recomputes it from the Normal font, so pictures over default rows are 12% short #715
• GT: assets/bugfixes/issue-620/gt.jpg
• Before: assets/bugfixes/issue-620/before.jpg
• After: assets/bugfixes/issue-620/after.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	Remaining: #713 — GT prints sheet 1 on 2 pages (tomcat right edge 580.95pt > printable 544.88pt); drawing extents never map to horizontal breaks
Element presence	Matches GT — all 6 elements present (GT carries tomcat/santa continuations on page 2, tracked under #713)
Position/size	Remaining: #715 — GT lays default rows out at 17pt vs our 15pt (clock height 132 vs 148pt); Remaining: #621 — Calibri default column 50.58 vs GT's measured 52.95pt/col; Remaining: #714 — declared cols widths still ignored on drawing-only sheets (one-factor probe run in this audit). The column-metric shrink this PR fixes is gone: tomcat x error −69.5pt → −19.0pt, clock width −16.5% → −4.6%
Rotation/flip	No deviation observed
Fill	Matches GT — text-box fill matches; ink delta fully accounted by the unclipped picture halves (#713)
Stroke/border	Matches GT — text-box border weight and all picture outline hairlines match at 150 DPI
Text content	Matches GT — both text lines identical
Font family/weight/style	Matches GT — Calibri regular both sides
Text color	Matches GT — black both sides
Alignment	Matches GT — both lines centred in the text box
Line/paragraph spacing	Remaining: #656 — text-box line advance 26.49pt vs GT 13.86pt (the xdr:txBody double-pitch defect), pushing line 2 below the frame
Clipping/overflow	Remaining: #713 — GT clips pictures at the printable edge and continues them on page 2; ours overflows the margin by 36.07pt

Checklist

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

🤖 Generated with Claude Code