Add Visio authoring and generated diagram assets

[paullizer]

Summary

Add authoring support for generated Visio diagrams and generated diagram assets, including creating Visio files and PNG renders for documentation or generated content.

User Value

Tracking this as a GitHub issue moves the backlog item out of a private markdown file and gives the team a clear place to prioritize, design, implement, and validate the work.

Proposed Behavior

Keep Visio ingestion separate from Visio generation. Avoid duplicating Mermaid support tracked in #718. Define generation, preview, export, and documentation usage behavior.

Acceptance Criteria

• The requested behavior is designed or implemented for the relevant SimpleChat surfaces.
• Permission, configuration, and existing-feature interactions are accounted for.
• Tests, documentation, or validation notes are added as appropriate.

Notes

• Source backlog entry: Visio And Mermaid Authoring Support
• Source file: priv-simplechat-2/grouped_feature_fix_backlog.md
• Backlog status: Extension / New
• Current anchors: docs/explanation/features/VISIO_INGESTION.md, application/single_app/functions_visio.py, markdown rendering/export surfaces
• Proposed roadmap priority: P2
• Proposed roadmap size: L
• Related: Add support for mermaid diagrams #718 already tracks Mermaid support. This issue should focus on Visio authoring and generated diagram assets without duplicating Mermaid scope.