fix(docx): preserve WPG grouped content

[developer0hye]

What changed

Preserve DOCX WordprocessingGroup (wpg) child shapes, text boxes, tables, and inline images that were previously dropped or partially parsed.

Why

docx-rs exposes only one child from a WPG group. The converter therefore lost sibling shapes and their text-box content. It also discarded group transforms, bodyPr text layout, and theme-backed colors.

Key changes

• scan WPG groups from word/document.xml in document order
• flatten nested group transforms into child positions and sizes
• preserve supported shape geometry, fills, strokes, rich text-box blocks, tables, and inline images
• retain text-box padding, vertical alignment, theme text colors, and geometry-aware text bounds
• add strict real-world fixture assertions for grouped shape count, geometry, text, table cells, and layout metadata

Verification

• cargo fmt --all -- --check
• cargo test -p office2pdf (1083 passed)
• cargo clippy -p office2pdf --all-targets -- -D warnings
• Microsoft Word GT compared visually against matched Before and After renders at 120 DPI

Visual comparison

Microsoft Word GT	Before	After

Related: #200
Related: #195