- Agents can send RenderMermaid with Mermaid source + canvas position
to create SVG texture overlays that coexist with other elements
- MermaidOverlay struct holds source, rendered SVG, SvgRenderer, and
lazy-loaded egui texture at a specific canvas position/size
- Server handles rendering via mermaid-rs, parses SVG for dimensions,
sends overlay data through DrawingCommand channel to GUI thread
- Canvas renders overlays as positioned textures between base SVG and
drawing elements, with proper pan/zoom transforms
- New MCP tool render_mermaid for agent access
- Overlays cleared on undo/redo/checkout to stay consistent with history
- 29 tests passing, clippy clean
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