fix(pptx): honor slide background for useBgFill

[developer0hye]

File submission policy

• Submitted evidence uses the existing public fixture and satisfies the submission policy.

Summary

PPTX shapes with p:sp/@useBgFill="1" now use the resolved slide background instead of the style a:fillRef color. The parser resolves the background before it finalizes slide, layout, and master shapes, and the regression covers both a focused style-fill case and the public issue fixture.

Related issue

Related: #1879

Testing

• cargo fmt --all -- --check — passed.
• git diff --check origin/main...HEAD — passed.
• Added parser regressions for a shape-level background fill and the issue fixture.
• cargo test --locked --workspace --profile ci — passed locally after the shape-finalization refactor.
• cargo clippy --locked --workspace --all-targets -- -D warnings — passed locally.
• Shape finalization reuses SlideParseContext to avoid duplicating its inputs.
• Integrated main 71e10311; hosted runtime CI passed on the matching integration tree. The fresh native PowerPoint layout, text, render and strict-cluster audit is complete.

Visual impact

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

Visual audit

• Issue: PPTX: a useBgFill shape paints its style fillRef colour instead of the slide background #1879
• Fixture: tests/fixtures/pptx/1-slide.pptx
• Page(s): 1
• Renderer and DPI: pdftoppm, 150 DPI
• Evidence mode: fix
• Layout audit report: assets/bugfixes/issue-1879/layout-audit.json
• Render cluster reports: assets/bugfixes/issue-1879/render-clusters-page-1.json
• Reference exporter differences: None
• Fine-detail threshold: 1pt
• 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
• Model vision findings: The previously purple panel now takes the white slide background, as p:sp/@useBgFill="1" specifies. PAGE 1, the right-side picture, both black rules, their flat endpoints, and the picture crop agree with native PowerPoint. The rules are solid 3.5pt and 1pt strokes (a:ln/@w=44450 and 12700); no dash, emphasis, rotation, clipping, or missing element was observed. Full pages, the pixel diff, and matched title/rule/picture crops show only boundary coverage and photo resampling fragments below the material-cluster floor. There are zero current material clusters.
• GT: assets/bugfixes/issue-1879/gt.jpg
• Before: assets/bugfixes/issue-1879/before.jpg
• After: assets/bugfixes/issue-1879/after.jpg
• Native: None

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
Element presence	Matches GT
Position/size	Matches GT
Rotation/flip	No deviation observed
Fill	Fixed — useBgFill panel is white like the slide background.
Stroke/border	Matches GT
Shape outline geometry	Matches GT
Text content	Matches GT
Font family/weight/style	Matches GT
Text color	Matches GT
Alignment	Matches GT
Line/paragraph spacing	Matches GT
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