feat: add boolean shape ops, visual undo tree, and Augmented Canvas branding

- Boolean operations (union, intersection, difference, xor) via i_overlay
  with Path shape rendering using earcutr triangulation
- Visual undo tree with branching history, checkout, and fork (Cmd+H)
  using Arc-based snapshots for structural sharing
- Consistent Augmented Canvas branding across app title, MCP server,
  CLI help text, and error messages
- macOS .app bundle script and Info.plist for Finder integration
- New MCP tool: boolean_op for agent-driven shape composition
- 26 tests passing (5 boolean, 6 history, 15 existing)

crates/agcanvas-mcp/src/bridge.rs

@@ -7,7 +7,7 @@ pub async fn send_request(ws_url: &str, request_json: &str) -> Result<String> {
         .await
         .map_err(|e| {
             anyhow::anyhow!(
-                "Cannot connect to agcanvas at {}. Is agcanvas running? Error: {}",
+                "Cannot connect to Augmented Canvas at {}. Is Augmented Canvas running? Error: {}",
                 ws_url,
                 e
             )