fix(xlsx): preserve fill boundary crossings

[developer0hye]

Fixes #1475

Summary

• precompute the positive-axis horizontal ink that owns each Excel cell junction after border conflict resolution
• start a later cell's right-side background bleed below that winning band while keeping the lower edge fixed
• preserve Excel's ordinary lower-left crossing ownership when a differently coloured upper-right fill starts at the same junction
• add focused regressions for the merged-title, ordinary adjacent-fill, and conditional-fill cases
• add fresh GT/before/after evidence plus 300-DPI layout and strict cluster reports

The page-1 title/body boundary now stays continuous at both pale-cell intersections. The same rule removes four page-2 conditional-fill notches without moving text or changing surrounding fill extents.

Related issue

Fixes #1475. Related: #982, #1472.

Testing

• cargo test -p office2pdf render::typst_gen::tests::table_codegen_tests::table_border_tests — 40 passed
• cargo test --workspace — passed, including 2,802 office2pdf library tests and all DOCX/PPTX/XLSX fixture suites
• python3 -m unittest discover -s scripts/tests — 326 passed
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo check --target wasm32-unknown-unknown -p office2pdf
• cargo check --target wasm32-unknown-unknown -p office2pdf --features wasm
• cargo check --target wasm32-unknown-unknown -p office2pdf --features wasm-cjk-font
• wasm-pack build crates/office2pdf --target web --features wasm
• qpdf --check target/issue-1475/after-final.pdf
• required pre-commit documentation freshness audit: PASS

Visual impact

• No rendered PDF change
• Rendered PDF change or visual evidence added
• Reason: The fix changes Excel fill-paint precedence at shared cell junctions.

Visual audit

• Issue: XLSX: a later body-cell fill cuts a notch through the title region's bottom rule #1475
• Fixture: Original Gift Budget and Tracker1.xlsx attachment from Gift Budget and Tracker1.xlsx misc issues #982, SHA-256 25f5dc75dab19ea12042979a61842314ddc226e3e45d447e36b2a2a104112613
• Page(s): 1-2
• Renderer and DPI: Poppler pdftoppm and MuPDF mutool at 300 DPI
• Evidence mode: fix
• Layout audit report: assets/bugfixes/issue-1475/layout-audit.json
• Render cluster reports: assets/bugfixes/issue-1475/render-clusters-page-1.json, assets/bugfixes/issue-1475/render-clusters-page-2.json
• Fine-detail threshold: 0.5pt
• Layout audit page count: Pass
• Layout audit text flow: Pass
• Layout audit visible fills: XLSX: adjacent differently colored fills overlap at their shared corner #1495
• Layout audit large shifts: Pass
• Layout audit fine shifts: XLSX: a header/footer run before the first font code takes the Normal font's family but not its size or colour #1210, XLSX: a merged centered title uses the nominal track width, seating it 1.04pt left of Excel #1493, XLSX: a three-line vertically centered cell seats its block 1pt high while the two-line peer matches #1494, XLSX: landscape sheet cell baselines share a 0.70pt downward seat #1496, XLSX: a two-row merged title block sits 1.14pt below Excel #1497, XLSX: a two-line centered table header compresses its line pitch #1498, XLSX chart: one middle value-axis tick sits 0.70pt above its peers #1499
• New follow-up issues found in this audit: XLSX: a merged centered title uses the nominal track width, seating it 1.04pt left of Excel #1493, XLSX: a three-line vertically centered cell seats its block 1pt high while the two-line peer matches #1494, XLSX: adjacent differently colored fills overlap at their shared corner #1495, XLSX: landscape sheet cell baselines share a 0.70pt downward seat #1496, XLSX: a two-row merged title block sits 1.14pt below Excel #1497, XLSX: a two-line centered table header compresses its line pitch #1498, XLSX chart: one middle value-axis tick sits 0.70pt above its peers #1499
• Model vision findings: Codex inspected the full GT and output pages, pixel diff, and all matched text and border crops at 300 DPI. The before crop visibly has two pale notches through the rose page-1 boundary; the after crop keeps that line continuous like GT. Page 2 retains two differently coloured E/F corner overlaps tracked in XLSX: adjacent differently colored fills overlap at their shared corner #1495, while the chart, images, table geometry, text order, fill regions, hairlines, and emphasis remain visually stable.
• GT: assets/bugfixes/issue-1475/gt.jpg
• Before: assets/bugfixes/issue-1475/before.jpg
• After: assets/bugfixes/issue-1475/after.jpg

Visual comparison

GT	Before	After

Fresh Poppler and MuPDF renders at 300 DPI both show the two page-1 pale notches in the before PDF and a continuous rose boundary in the after PDF.

• layout audit: page 1 = 12/12 lines matched; page 2 = 34/34; no missing/extra text, wrap/reflow, visibility mismatch, or >=5pt shift
• page-1 strict report: 100/100 clusters dispositioned
• page-2 strict report: 101/101 clusters dispositioned

The page-2 audit retains two 1pt adjacent-colour corner differences, both tracked in #1495. Before this change page 2 had six visible-fill mismatches, so four are fixed and none are newly introduced.

The PDF text-layer check reports two extra GT spaces because the native exporter fragments the source phrase Birthday Budget as Birt hday Bu dget. The output preserves the workbook source phrase, so this is a GT text-fragmentation artifact rather than output content loss.

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, 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 printable worksheets in workbook order.
Element presence	Matches GT — all 46 audited text lines, chart, images, table regions, and footer elements are present.
Position/size	Remaining: #1210, #1493, #1494, #1496, #1497, #1498, #1499. Both #1475 crossings and four page-2 conditional-fill crossings are fixed without changing affected sizes.
Rotation/flip	Matches GT — no rotated or flipped mismatch.
Fill	Remaining: #1495. The rose page-1 boundary is fixed; two adjacent-colour page-2 corners remain.
Stroke/border	Matches GT — page 1 separator is the fill boundary fixed here; page 2 chart gridlines, plot line/markers, and table seams match in position and pattern.
Shape outline geometry	Matches GT — chart, images, and table geometry remain unchanged.
Text content	Matches GT — output preserves the workbook source; only the native GT text layer inserts two fragment spaces.
Font family/weight/style	Remaining: #1210. Regular title/body runs and bold conditional category labels otherwise match; no italic or underline runs are missing.
Text color	Remaining: #1210. Other text colours match GT.
Alignment	Remaining: #1493. Other alignment matches GT.
Line/paragraph spacing	Remaining: #1494, #1498. Other line and paragraph spacing matches GT.
Clipping/overflow	Matches GT — no new clipping, overflow, wrap, or reflow.

Checklist

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