feat(xlsx): print row and column heading gutters when printOptions asks

[developer0hye]

Summary

<printOptions headings="1"/> prints Excel's row-number gutter and column-letter strip; the attribute was never read, so both gutters were missing from every page.

Measured off the native Excel GT (both NumberFormatTests sheets, agreeing across Letter/A4 and 1-vs-3-digit row counts): the gutter is a fixed 22pt interior + 1pt black separator (it does not scale with digit count), the strip 12pt + separator; heading cells are unfilled with constant-#565656 1pt rules between them (the PDF "gradients" are degenerate single-colour shadings); a 1pt black frame encloses the block on every page, shrinking to content on the partial last page; digits/letters are the Normal font, regular, centred horizontally, seated 3pt above the band bottom.

Implementation materialises the gutter as a real prepended column (row numbers flow with Typst pagination, width packing, merges and drawings by construction) and the strip as rows[0] re-emitted as a repeating header above print-title rows. The strip's bottom boundary joins the repeating-header tie rules (a body top border cannot strip it from pages 2+); gridline seeding (#622) skips the frame-owned exterior; width pagination packs the first column group against the full printable width; non-left-anchored print-title columns keep their repeat (gutter yields on overflow pages — Excel repeats both; TODO documents the multi-range requirement).

Related issue

Fixes #623

Testing

• cargo test --workspace — green (2100+ tests; ~25 new red-first tests across scanner, heading materialisation, base-26 letters, pagination slices, codegen repeat blocks, frame bands, descender seat, tie adoption, DOCX/PPTX guards)
• cargo +1.97 clippy --workspace --all-targets — clean; cargo fmt --check — clean
• GT parity: separator/frame bands on GT's exact boundaries; heading baselines within 0.10pt of the measured seat; row numbers continue to 337 across pages; letters + top frame edge repeat on every flowed page

Visual impact

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

Visual audit

• Issue: XLSX: printOptions headings="1" is ignored, so the row-number and column-letter gutters never print #623
• Fixture: tests/fixtures/xlsx/poi/NumberFormatTests.xlsx
• Page(s): Tests sheet page 1 (output page 2; GT page dimensions differ per open XLSX: an omitted pageSetup paperSize falls back to A4 instead of the schema-default Letter, so a Letter workbook prints 16.7pt narrow and 49.9pt tall #717)
• Renderer and DPI: pdftoppm, 150 DPI
• Evidence mode: fix
• New follow-up issues found in this audit: XLSX: the print-heading frame leaves the gutter's bottom span open at Typst-chosen page breaks, so intermediate pages are not fully framed #722
• GT: assets/bugfixes/issue-623/gt.jpg
• Before: assets/bugfixes/issue-623/before.jpg
• After: assets/bugfixes/issue-623/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: #658 — 12pt vs GT 13pt rows; Remaining: #717 — A4 vs GT's schema-default Letter; sheet order matches
Element presence	Fixed — gutter, letter strip, corner box, inter-heading rules and the outer frame are all present (before: entirely absent); Remaining: #718 — the formatted empty column E and its letter are absent
Position/size	Remaining: #722 — intermediate Typst-flowed pages leave the gutter's bottom frame span open; Remaining: #658 — gutter rule pitch follows the 12pt row track. The heading geometry this PR adds matches GT (22pt+1pt gutter, 12pt+1pt strip, separators at identical device positions)
Rotation/flip	No deviation observed
Fill	Matches GT — heading bands and corner unfilled white; body header band identical
Stroke/border	Fixed — black separators, gray #565656 inter-heading rules, and the outer frame land on GT's bands; Remaining: #722 — the intermediate-page bottom frame gap over the gutter span
Text content	Fixed — row numbers and letters present and correct; Remaining: #719 — body cells drop xml:space leading spaces
Font family/weight/style	Matches GT — regular-weight Normal font digits/letters, ink heights within 1px
Text color	Matches GT — black
Alignment	Fixed — digits centred and bottom-seated within 0.10pt of GT's 3pt seat; Remaining: #657 — body-cell left inset 1pt narrow
Line/paragraph spacing	Remaining: #658 — 12pt vs 13pt row pitch
Clipping/overflow	No deviation observed

Checklist

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

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