Headless text and SVG

A pure Dart DiagramController can create and edit documents without Flutter. Approximate text metrics are the default headless fallback. Supply a DiagramTextLayoutEngine for font-aware geometry before exporting.

The SVG codec consumes resolved text geometry. A mounted Flutter canvas supplies FlutterDiagramTextLayoutEngine. For unmounted Flutter export, install an extension carrying it: DiagramExtension(id: 'app.backends', textLayout: FlutterDiagramTextLayoutEngine(), output: FlutterDiagramOutput()); both backends are in package:vyuh_diagram_flutter/extension_api.dart. In pure Dart, PangoTextLayoutExtension(fontFiles: ..., defaultFontFamily: ...) from package:vyuh_diagram_core/vyuh_diagram_pango.dart supplies font-aware metrics, and SvgExtension() from package:vyuh_diagram_codec_svg/vyuh_diagram_codec_svg.dart supplies SVG output. Load the intended fonts before creating or refreshing the controller. Native text shaping requires the platform dependencies described in the core package's native text documentation.

See output for export APIs and controller for ownership.