fix(pptx): round wrapped lines on the text story grid

[developer0hye]

Summary

Round each wrapped PowerPoint line's running baseline within its text story,
retaining the fractional phase until that line is painted. Startup continuation
lines now match native PowerPoint within 0.12pt; product continuations move from
179.0/223.0pt to 178.6/223.6pt (native: 178.56/223.68pt).

Use each physical line's largest font size when restoring mixed-size baseline
seats. Native 90% and 100% spacing probes agree within 0.103pt. Move bullets,
links, and underlines with their associated lines while preserving Typst's
wrapping and layout heights, paragraph-end font handling, and fractional
story origins. Top, center, and bottom anchors are covered.

Related issue

Related: #1584. Remaining independent defects: #1581 and #1582.

Testing

• cargo test --locked --workspace --no-fail-fast: passed (3487 tests; 15 ignored).

• All 87 fixed-page text-box tests passed, including RED/GREEN physical-line
  rounding and mixed-size regressions using embedded fonts.

• Link/underline line-seating regression passed with independent line movements.

• cargo clippy --locked --workspace --all-targets -- -D warnings: passed.

• cargo fmt --all --check and git diff --check: passed.

• Thirteen-page native comparison: layout audit passes at 0.5pt; searchable
  text matches (2,184 normalized codepoints, identical codepoint-class census).

• All thirteen strict cluster reports pass; 27 clusters have explicit dispositions.

Visual impact

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

Visual audit

• Issue: PPTX: wrapped continuation baselines retain unsnapped fractional line offsets #1584
• Fixture: Four public PPTX fixtures under tests/golden_mocks/business/sources/pptx/: 01_startup_pitch_en.pptx, 03_product_launch_en.pptx, 08_marketing_report_en.pptx, 09_lecture_ko.pptx, followed by startup slide 2 center/bottom anchor probes. Mapping, reproduction, and hashes: assets/bugfixes/issue-1584/README.md.
• Page(s): 1-13
• Renderer and DPI: pdftoppm, 150 DPI; native PowerPoint 16.112.3 GT
• Evidence mode: fix
• Layout audit report: assets/bugfixes/issue-1584/layout-audit.json
• Render cluster reports: assets/bugfixes/issue-1584/render-clusters-page-1.json, assets/bugfixes/issue-1584/render-clusters-page-2.json, assets/bugfixes/issue-1584/render-clusters-page-3.json, assets/bugfixes/issue-1584/render-clusters-page-4.json, assets/bugfixes/issue-1584/render-clusters-page-5.json, assets/bugfixes/issue-1584/render-clusters-page-6.json, assets/bugfixes/issue-1584/render-clusters-page-7.json, assets/bugfixes/issue-1584/render-clusters-page-8.json, assets/bugfixes/issue-1584/render-clusters-page-9.json, assets/bugfixes/issue-1584/render-clusters-page-10.json, assets/bugfixes/issue-1584/render-clusters-page-11.json, assets/bugfixes/issue-1584/render-clusters-page-12.json, assets/bugfixes/issue-1584/render-clusters-page-13.json
• Reference exporter differences: None
• Fine-detail threshold: 0.5pt
• Layout audit page count: Pass
• Layout audit text flow: Pass
• Layout audit visible fills: Pass
• Layout audit rectangle geometry: Pass
• Layout audit large shifts: Pass
• Layout audit fine shifts: Pass
• New follow-up issues found in this audit: None; independent residuals remain tracked in PPTX: multiplication sign disables advance-grid rounding for the entire Latin run #1581 and PPTX: word-level width rounding drops kerning across spaces #1582.
• Model vision findings: Full pages, pixel diffs, and full-scale bands confirm corrected startup/product continuation positions, unchanged wrapping and markers, and preserved top/center/bottom placement. Korean bold/regular emphasis, chart axes, ticks, gold bar edges, and legend outline remain intact. Pages 2, 6, 11, 12, and 13 were freshly inspected; the other eight pages have identical decoded GT/output/diff/band pixels to the fully inspected PPTX: top-anchored text snaps to page coordinates instead of its text-box origin #1583 comparison. The remaining 17 page-5 suffix clusters and seven page-9 horizontal-drift clusters are tracked in PPTX: word-level width rounding drops kerning across spaces #1582 and PPTX: multiplication sign disables advance-grid rounding for the entire Latin run #1581. One gold legend-edge resampling cluster and two regular Korean glyph-edge rasterization clusters retain matching geometry and emphasis.
• GT: assets/bugfixes/issue-1584/gt.jpg
• Before: assets/bugfixes/issue-1584/before.jpg
• After: assets/bugfixes/issue-1584/after.jpg
• Native: None
• Compare: assets/bugfixes/issue-1584/compare.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, rectangle geometry deviation, 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: thirteen pages in documented fixture/probe order.
Element presence	Matches GT: titles, body lines, markers, shapes, and pictures remain present.
Position/size	Remaining: #1581 and #1582; separate Latin advance-grid and cross-space kerning defects.
Rotation/flip	Matches GT.
Fill	Matches GT: covers, cards, bars, and Korean panel retain colors.
Stroke/border	Matches GT: chart axes, ticks, gold bar edges, and legend outline remain present.
Shape outline geometry	Matches GT: no material matched rectangle or outline deviation.
Text content	Matches GT: searchable text and codepoint-class census match.
Font family/weight/style	Matches GT: regular bodies, bold titles, and Korean emphasis remain intact.
Text color	Matches GT.
Alignment	Matches GT: story anchors and paragraph starts are preserved.
Line/paragraph spacing	Fixed: each continuation uses the running story grid and its own font-size seat; wrapping and paragraph layout height are unchanged.
Clipping/overflow	No deviation observed.

Checklist

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