Add Visio authoring and generated diagram assets

@paullizer is already working on this.

Since Jun 26, 2026.

## 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: #718 already tracks Mermaid support. This issue should focus on Visio authoring and generated diagram assets without duplicating Mermaid scope.