PPTX: render SmartArt drawing content

[developer0hye]

Problem

SmartArt drawing content is omitted entirely. PowerPoint renders five blue blocks in the public fixture, while office2pdf produces a blank slide.

Evidence

Microsoft PowerPoint GT	office2pdf Before

Expected

Render the cached SmartArt drawing when present and retain a structured fallback when no drawing cache is available.

Related: #213
Discovered by: #216

[developer0hye]

Fixed by PR #325 (merged).

SmartArt now renders its pre-rendered drawing cache (diagrams/drawingN.xml) when present — resolved through the slide rels → data model → dataModelExt relId → drawing part — parsing each <dsp:sp> (geometry, theme-resolved fill/outline, text) into shape and text-box elements positioned relative to the diagram frame, with the structured node list as a fallback. Verified on the SmartArt fixture: the five blue blocks render in PowerPoint's 2/2/1 layout with the accent fill (see the PR's GT/before/after), where it was previously a blank slide.