fix(xlsx): center filled merged titles on visible band

[developer0hye]

Summary

• center an Excel filled horizontal merge on the visible boundary-band region
• apply the seat to the placed spill line without changing its measurement region
• keep unfilled, left-aligned, non-Excel, and unshifted spill-cell output unchanged
• add an actual-layout regression that varies fill and center/left alignment

Related issue

Fixes #1493

Testing

• cargo test -p office2pdf a_centered_merged_fill_uses_the_excel_background_band_for_its_text_seat -- --nocapture
• cargo test -p office2pdf spill_cell -- --nocapture
• cargo test -p office2pdf a_spill_clip_box_stops_at_the_cell_edge_its_anchor_faces -- --nocapture
• cargo test --workspace --quiet
• python3 -m unittest discover -s scripts/tests (326 tests)
• 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-1493/final.pdf
• fresh Poppler and MuPDF 300-DPI full-page, diff, and matched-crop inspection

Visual impact

• No rendered PDF change
• Rendered PDF change or visual evidence added
• Reason: the page-1 merged title moves 1pt right to Excel's visible-region center.

Visual audit

• Issue: XLSX: a merged centered title uses the nominal track width, seating it 1.04pt left of Excel #1493
• Fixture: Gift Budget and Tracker1.xlsx (25f5dc75dab19ea12042979a61842314ddc226e3e45d447e36b2a2a104112613)
• Page(s): 1-2
• Renderer and DPI: Poppler and MuPDF, 300 DPI
• Evidence mode: fix
• Layout audit report: assets/bugfixes/issue-1493/layout-audit.json
• Render cluster reports: assets/bugfixes/issue-1493/render-clusters-page-1.json, assets/bugfixes/issue-1493/render-clusters-page-2.json
• Fine-detail threshold: 0.5pt
• Layout audit page count: Pass
• Layout audit text flow: Pass
• Layout audit visible fills: Pass
• 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 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: None
• Model vision findings: On page 1, the title now starts at 110.958pt against Excel's 111.000pt while retaining its 110.000pt baseline, shaped width, and rose-panel bounds. The three 1pt-high body lines remain XLSX: a three-line vertically centered cell seats its block 1pt high while the two-line peer matches #1494 and the footer styling remains XLSX: a header/footer run before the first font code takes the Normal font's family but not its size or colour #1210. Page 2 is pixel-identical before/after this fix; its visible residuals remain XLSX: a header/footer run before the first font code takes the Normal font's family but not its size or colour #1210 and XLSX: landscape sheet cell baselines share a 0.70pt downward seat #1496-XLSX chart: one middle value-axis tick sits 0.70pt above its peers #1499. Full pages, 5% diff images, every fine-shift crop, title crops, hairlines, and emphasis were inspected at 300 DPI.
• GT: assets/bugfixes/issue-1493/gt.jpg
• Before: assets/bugfixes/issue-1493/before.jpg
• After: assets/bugfixes/issue-1493/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
• 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
Element presence	Matches GT
Position/size	Remaining: #1210, #1494, #1496, #1497, #1498, #1499; Fixed #1493
Rotation/flip	Matches GT
Fill	Matches GT
Stroke/border	Matches GT
Shape outline geometry	Matches GT
Text content	Matches GT
Font family/weight/style	Remaining: #1210
Text color	Remaining: #1210
Alignment	Fixed #1493
Line/paragraph spacing	Remaining: #1494, #1497, #1498
Clipping/overflow	Matches GT

Checklist

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