Add Vitest suites for relations, properties, catalogue, symbols, edge
styles, flow solver, settings, JointJS shapes/serialization and the
controller lifecycle (add/connect/edit/delete, undo-redo, copy-paste,
flow, save-load). Introduce headless DiagramController (Paper-less) so the
controller is testable outside a browser, and fix the flow direction sign.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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