feat: add Mermaid overlay support for agents to inject positioned diagrams

- 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

crates/agcanvas-mcp/src/tools.rs

@@ -187,9 +187,27 @@ pub struct BooleanOpParam {
     pub stroke_width: Option<f32>,
 }
 
+#[derive(Debug, serde::Deserialize, schemars::JsonSchema)]
+pub struct RenderMermaidParam {
+    #[schemars(description = "Session ID to target. If omitted, uses the active session.")]
+    pub session_id: Option<String>,
+    #[schemars(description = "Mermaid diagram source code (e.g., 'flowchart LR\\n    A-->B')")]
+    pub mermaid_source: String,
+    #[schemars(description = "X position on canvas (default: 0)")]
+    pub x: Option<f32>,
+    #[schemars(description = "Y position on canvas (default: 0)")]
+    pub y: Option<f32>,
+    #[schemars(description = "Override width (default: natural SVG width)")]
+    pub width: Option<f32>,
+    #[schemars(description = "Override height (default: natural SVG height)")]
+    pub height: Option<f32>,
+}
+
 #[derive(Debug, serde::Deserialize, schemars::JsonSchema)]
 pub struct BatchParam {
-    #[schemars(description = "JSON array of request objects. Each object must have a 'type' field and the same parameters as individual tool calls. Example: [{\"type\": \"CreateDrawingElement\", \"shape_type\": \"rectangle\", \"x\": 0, \"y\": 0, \"width\": 100, \"height\": 100}, {\"type\": \"CreateDrawingElement\", \"shape_type\": \"ellipse\", \"center_x\": 200, \"center_y\": 200, \"radius_x\": 50, \"radius_y\": 50}]")]
+    #[schemars(
+        description = "JSON array of request objects. Each object must have a 'type' field and the same parameters as individual tool calls. Example: [{\"type\": \"CreateDrawingElement\", \"shape_type\": \"rectangle\", \"x\": 0, \"y\": 0, \"width\": 100, \"height\": 100}, {\"type\": \"CreateDrawingElement\", \"shape_type\": \"ellipse\", \"center_x\": 200, \"center_y\": 200, \"radius_x\": 50, \"radius_y\": 50}]"
+    )]
     pub requests_json: String,
 }
 
@@ -527,6 +545,26 @@ impl AgCanvasServer {
         self.call_agcanvas(&request).await
     }
 
+    #[tool(
+        name = "render_mermaid",
+        description = "Render a Mermaid diagram (flowchart, sequence, etc.) as an SVG overlay at a specific position on the canvas. The diagram appears as a visual element that can coexist with other shapes and diagrams."
+    )]
+    async fn render_mermaid(
+        &self,
+        Parameters(params): Parameters<RenderMermaidParam>,
+    ) -> Result<CallToolResult, McpError> {
+        let request = serde_json::json!({
+            "type": "RenderMermaid",
+            "session_id": params.session_id,
+            "mermaid_source": params.mermaid_source,
+            "x": params.x,
+            "y": params.y,
+            "width": params.width,
+            "height": params.height,
+        });
+        self.call_agcanvas(&request).await
+    }
+
     #[tool(
         description = "Send multiple Augmented Canvas operations in one request. Accepts a JSON array of request objects and returns one response per operation in a BatchResult."
     )]