fix(xlsx): seat centered multi-row text blocks

[developer0hye]

Summary

• seat vertically centred XLSX cells spanning fixed-height rows against the full joined track instead of Typst's text-metric box
• preserve the existing one-row, bottom-aligned multi-row, top-aligned, auto-row, and non-spreadsheet paths
• reproduce Excel's lower-half resolution of a symmetric multi-row centre on its 1/300-inch PDF position grid
• add a compiled two-row/two-line regression that uses a portable embedded font and automatic wrapping

Related issue

Fixes #1497

Testing

• cargo test --locked -p office2pdf centered_two_row_sheet_merge_uses_the_full_fixed_track_seat -- --nocapture
• cargo test --locked -p office2pdf sheet_cell -- --nocapture (28/28)
• cargo test --locked -p office2pdf row_spanning_cell_keeps_its_declared_alignment_in_a_tight_row -- --nocapture
• cargo test --locked -p office2pdf centered_fixed_sheet_rows_share_one_center_across_two_and_three_wrapped_lines -- --nocapture
• full workspace coverage: 2,806 library tests, DOCX/PPTX/XLSX/public-visual/PDF/CLI/package integration suites, and doctests passed across the workspace run and explicit remaining-suite runs
• the first contended workspace run reached perf_medium_xlsx_conversion_under_30_seconds at 33.52s after the long rendering suites; its unchanged isolated rerun passed at 12.933s, and no threshold or fixture was changed
• python3 -m unittest discover -s scripts/tests -p 'test_*.py' (326 tests; 30 business mocks and stored layout baseline included)
• cargo fmt --check
• git diff --check
• cargo check --locked --workspace
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo check --locked --target wasm32-unknown-unknown -p office2pdf
• cargo check --locked --target wasm32-unknown-unknown -p office2pdf --features wasm
• cargo check --locked --target wasm32-unknown-unknown -p office2pdf --features wasm-cjk-font
• wasm-pack build crates/office2pdf --target web --features wasm
• wasm-pack build crates/office2pdf --target web --features wasm-cjk-font
• Node WASM tests with CARGO_PROFILE_DEV_DEBUG=0: 10/10 general and 11/11 CJK passed
• exact-main versus branch rendering of all 10 business XLSX fixtures (17 pages): 9 PDFs byte-identical; 08_budget_ko.xlsx changes only its two vertically centred multi-row merge labels by +0.1366pt, toward the stored golden positions, with unchanged x/width/wraps/fills/100 rectangles; all 20 PDFs pass qpdf --check
• qpdf --check target/issue-1497/after.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-2 two-line title moves 1.1384pt upward onto Excel's baseline while its x origin, width, line pitch, merged-cell geometry, and every other page element remain unchanged. Page 1 is byte- and layout-identical.

Visual audit

• Issue: XLSX: a two-row merged title block sits 1.14pt below Excel #1497
• Fixture: Gift Budget and Tracker1.xlsx (25f5dc75dab19ea12042979a61842314ddc226e3e45d447e36b2a2a104112613)
• GT PDF: ed705d89307735651eb67abedb59d014666b2567aa3be73eabd7f4518dd8f659
• After PDF: 8e014bf9316ba4259f9e8e55c9afe6facbbba0e8a460b5e2a49da9ff7632ad9e
• Page(s): 1-2
• Renderer and DPI: Poppler and MuPDF, 300 DPI
• Evidence mode: fix
• Layout audit report: assets/bugfixes/issue-1497/layout-audit.json
• Render cluster reports: assets/bugfixes/issue-1497/render-clusters-page-1.json, assets/bugfixes/issue-1497/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 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: Both title baselines change from +1.1384pt below Excel to within 0.00002pt of Excel. Page 1 retains all 12 text items and 45 rectangles; page 2 retains all 22 text items and 156 rectangles. The title's x origins, widths, two-line pitch, font emphasis, fill, and surrounding geometry are unchanged. Poppler and MuPDF matched crops agree. All 73 page-1 and 50 page-2 strict 5% diff clusters were inspected and dispositioned. The four remaining text shifts over 0.5pt are the two table-header lines (XLSX: a two-line centered table header compresses its line pitch #1498), chart $140 tick (XLSX chart: one middle value-axis tick sits 0.70pt above its peers #1499), and sensitivity footer (XLSX: a header/footer run before the first font code takes the Normal font's family but not its size or colour #1210).
• GT: assets/bugfixes/issue-1497/gt.jpg
• Before: assets/bugfixes/issue-1497/before.jpg
• After: assets/bugfixes/issue-1497/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, #1498, #1499; Fixed #1497
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	Remaining: #1499; Fixed #1497
Line/paragraph spacing	Remaining: #1498
Clipping/overflow	Matches GT

GT integrity

• PDF pages: 2
• Printable worksheets: 2
• Hidden worksheets: 1
• Structural corruption: none observed
• GT-side font substitution: Arial only; the corrected title uses Century Gothic
• Source inspection records Century Gothic 30pt in B2:D3 over two declared 49.5pt rows; this corrects the original issue body's 40pt description without changing the measured acceptance target

Checklist

• Commit includes a Signed-off-by line
• Mandatory documentation freshness reviewer returned PASS: before commit
• PR scope contains one root cause
• Remaining visual deviations each reference an open issue