fix(pptx): retain fractional text-story origins

[developer0hye]

Summary

Preserve fractional text-box origins when seating top-anchored PowerPoint
paragraphs. A frame at 144pt with a 3.6pt inset now retains its 147.6pt story
origin: the lecture heading moves from 163pt to 162.6pt (native: 162.48pt),
and later paragraphs and bullets follow the same local grid. All affected
paragraph starts agree with native within 0.12pt.

Native quarter-point translation probes distinguish story-relative rounding
from page rounding. The fix captures the story origin for top anchors and
passes it through the existing paragraph and marker helpers. Typst still
chooses wrapping and layout height.

Related issue

Related: #1583. Remaining independent defects: #1581, #1582, #1584.
The separate physical-line rounding defect in #1584 remains visible after
correcting paragraph origins: product continuations move from −0.16pt to
+0.44pt and from −0.28pt to −0.68pt relative to native. This PR records those
residuals explicitly and keeps the root causes separate.

Testing

• cargo test --locked --workspace --no-fail-fast: passed (3485 tests; 15 ignored).
• cargo clippy --locked --workspace --all-targets -- -D warnings: passed.
• cargo fmt --all -- --check and git diff --check: passed.
• RED/GREEN translation regression: three plain paragraphs and three bullets,
  two embedded font families/sizes, and 0.25/0.50/0.75pt translations.
• All 85 fixed-page text-box tests passed.
• Eleven-page native comparison; searchable text matches (1,608 normalized
  codepoints, identical codepoint census).

Visual impact

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

Visual audit

• Issue: PPTX: top-anchored text snaps to page coordinates instead of its text-box origin #1583
• Fixture: Four public PPTX fixtures in 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; concatenated in that order. Full mapping and hashes: assets/bugfixes/issue-1583/README.md.
• Page(s): 1-11
• Renderer and DPI: pdftoppm, 150 DPI; native PowerPoint 16.112.3 GT
• Evidence mode: fix
• Layout audit report: assets/bugfixes/issue-1583/layout-audit.json
• Render cluster reports: assets/bugfixes/issue-1583/render-clusters-page-1.json, assets/bugfixes/issue-1583/render-clusters-page-2.json, assets/bugfixes/issue-1583/render-clusters-page-3.json, assets/bugfixes/issue-1583/render-clusters-page-4.json, assets/bugfixes/issue-1583/render-clusters-page-5.json, assets/bugfixes/issue-1583/render-clusters-page-6.json, assets/bugfixes/issue-1583/render-clusters-page-7.json, assets/bugfixes/issue-1583/render-clusters-page-8.json, assets/bugfixes/issue-1583/render-clusters-page-9.json, assets/bugfixes/issue-1583/render-clusters-page-10.json, assets/bugfixes/issue-1583/render-clusters-page-11.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: PPTX: wrapped continuation baselines retain unsnapped fractional line offsets #1584
• New follow-up issues found in this audit: None; residuals are already tracked in PPTX: multiplication sign disables advance-grid rounding for the entire Latin run #1581, PPTX: word-level width rounding drops kerning across spaces #1582, PPTX: wrapped continuation baselines retain unsnapped fractional line offsets #1584.
• Model vision findings: Pages 5, 6 and 11 were freshly inspected as full GT/output pages, diffs and matched full-scale bands. The other eight pages have identical paint traces and decoded GT/output/diff/band pixels to the fully inspected PPTX: a wrapped slide paragraph seats its first baseline 0.96pt above where PowerPoint does #1177 merge comparison. All 27 current material-cluster crops and both fine-shift crops were inspected again. Top-anchored paragraph origins now agree with native within 0.12pt. Covers retain title weights and fills; cards, chevrons, charts and the Korean panel retain their shapes and content. Thin axes, ticks, bar edges and legend outlines remain present. Page 5's 17 suffix clusters remain PPTX: word-level width rounding drops kerning across spaces #1582; page 9's seven width-drift clusters remain PPTX: multiplication sign disables advance-grid rounding for the entire Latin run #1581. Three clusters show a resampled gold legend-key edge or regular Korean glyph-edge rasterization with matching geometry and emphasis. Continuation-line offsets on pages 2 and 6 remain PPTX: wrapped continuation baselines retain unsnapped fractional line offsets #1584. JPEG text was inspected at full scale after encoding.
• GT: assets/bugfixes/issue-1583/gt.jpg
• Before: assets/bugfixes/issue-1583/before.jpg
• After: assets/bugfixes/issue-1583/after.jpg
• Native: None

Visual comparison

Each image contains the eleven full pages in collection order.

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: all eleven pages in the documented fixture order.
Element presence	Matches GT: text, markers, shapes and pictures remain present.
Position/size	Remaining: #1581, #1582, #1584; advance-grid, cross-space kerning and continuation-line rounding respectively.
Rotation/flip	Matches GT.
Fill	Matches GT: covers, cards, chart fills and Korean panel.
Stroke/border	Matches GT: thin axes, ticks, legend outlines and bar edges remain present.
Shape outline geometry	Matches GT.
Text content	Matches GT: normalized searchable text and codepoint census agree.
Font family/weight/style	Matches GT: same title/body weights and Korean emphasis in full-scale crops.
Text color	Matches GT.
Alignment	Fixed: paragraphs and bullets retain the fractional story origin.
Line/paragraph spacing	Remaining: #1584; wrapped continuation-line rounding.
Clipping/overflow	Matches GT on the original eleven-page fix collection.

Checklist

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