fix(docx): render paragraph shading

[developer0hye]

Summary

Paragraph-level shading (w:pPr/w:shd) was dropped: the CLI-manual fixture's shaded monospace code blocks rendered on plain white, indistinguishable from body text. docx-rs only parsed run and table-cell shading, so the docx-rs patch branch now reads paragraph shading (developer0hye/docx-rs#2, merged). office2pdf carries the fill through the IR (ParagraphStyle.background, style merge included) and paints it on the paragraph's full-width block wrapper.

Related issue

Fixes #351

Testing

• cargo test --workspace — all green (perf suite re-run in isolation)
• New tests: test_paragraph_shading_extracted_as_background (parser), test_generate_paragraph_with_background_shading (codegen), plus ParagraphStyle::merge_from coverage; docx-rs gains test_read_paragraph_shading
• Converted 05_technical_manual_en.docx: all four code blocks now carry the light-gray background, matching the Word GT at 150 DPI

Visual impact

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

Visual audit

• Issue: DOCX: paragraph shading (w:shd) is not rendered #351
• Fixture: 05_technical_manual_en.docx (synthetic real-world audit set, external)
• Page(s): 1
• Renderer and DPI: pdftoppm, 150 DPI
• Evidence mode: fix
• New follow-up issues found in this audit: None
• GT: assets/bugfixes/issue-351/gt.jpg
• Before: assets/bugfixes/issue-351/before.jpg
• After: assets/bugfixes/issue-351/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	Matches GT
Element presence	Fixed (code-block shading present)
Position/size	Matches GT
Rotation/flip	No deviation observed
Fill	Fixed
Stroke/border	No deviation observed
Text content	Matches GT (double hyphens verbatim since #381)
Font family/weight/style	No deviation observed
Text color	No deviation observed
Alignment	Matches GT
Line/paragraph spacing	Remaining: #354 (line pitch still larger than Word)
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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