fix(xlsx): overlay same-row worksheet drawings

[developer0hye]

Summary

• emit one reserved flow box per worksheet anchor row instead of one per shape
• size that box to the tallest shape on the row and place each shape at its own horizontal offset
• drop generate_sheet_anchor, whose placed-drawing arms became dead

Related issue

Fixes #459

Details

Excel lays shapes anchored to the same row out side by side over the grid. office2pdf reserved a full-width flow box per shape, so each one advanced the flow before the next.

Shape top coordinate at 150 DPI:

Shape	Excel GT	before	after
accent1	188	246	246
accent1 lighter 60	188	486	246
accent6 shade 75	188	726	246

The tripled reservation also spilled the sheet onto a blank second page; the workbook now exports to one page, as Excel does. The remaining offset from Excel's 188 px, and the undersized shape bounds, are the anchor metrics tracked in #460.

Testing

• cargo fmt --all -- --check
• cargo test --workspace
• cargo clippy --workspace --all-targets --all-features
• pdfinfo page-count comparison
• pdftoppm -r 150 rendering, fill-bounds sampling, and magick compare -metric AE -fuzz 5%

Visual impact

• No rendered PDF change
• Rendered PDF change or visual evidence added
• Reason: same-row worksheet drawings no longer stack, and the sheet no longer gains a blank page.

Visual audit

• Issue: XLSX: same-row worksheet drawings stack vertically and spill onto a blank page #459
• Fixture: tests/fixtures/xlsx/theme_color_drawing.xlsx
• Page(s): page 1 of the Excel GT and of the office2pdf output
• Renderer and DPI: pdftoppm, 150 DPI
• Evidence mode: fix
• New follow-up issues found in this audit: None
• GT: assets/bugfixes/issue-459/gt.jpg
• Before: assets/bugfixes/issue-459/before.jpg
• After: assets/bugfixes/issue-459/after.jpg

The GT is the native Microsoft Excel export already captured for this fixture in #434, reused unchanged: same workbook, same page, same renderer, same 150 DPI.

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

The 5% fuzz sweep against the size-normalized GT falls from 290,500 to 121,943 differing pixels. Every remaining cluster is the residual shape displacement and undersizing tracked in #460. The fixture has no hairlines, borders, or dashed strokes, and every run is regular weight with no italic or underline.

Deviation audit

Check	Result
Page count/order	Fixed - the workbook exports to one page, as Excel does; the blank second page is gone
Element presence	Matches GT - the title and all three drawing labels are present
Position/size	Fixed - the three shapes now share one top coordinate instead of stacking at 246/486/726 px; Remaining: #460 for the residual offset from Excel's 188 px and the undersized bounds
Rotation/flip	No deviation observed - the fixture contains no rotated or flipped elements
Fill	Matches GT - all three RGB values match Excel exactly after #430
Stroke/border	No deviation observed - the shapes intentionally have no outline or hairline
Text content	Matches GT - all four strings are preserved
Font family/weight/style	Matches GT - the labels render Calibri after #461 and the title the workbook Normal font
Text color	Matches GT - all text remains black
Alignment	Matches GT - drawing labels remain horizontally and vertically centered
Line/paragraph spacing	No deviation observed - each text region is one regular single-line paragraph
Clipping/overflow	Fixed - the flow no longer reserves three shape heights, so nothing spills past the page

Checklist

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