feat: Scaffold Dual-editor package with resizable split-view layout #225

Description

Language service spike: #209

Create packages/dual-editor (@openworkflowspec/dual-editor) and implement the resizable split-view layout inside it - text editor pane on one side, the existing diagram pane on the other, with a draggable divider.

Layout and shell only: no new editing logic, no parsing, no Monaco configuration. Wiring text edits through to the diagram is #226.

The package depends on both @openworkflowspec/text-editor and @openworkflowspec/diagram-editor; neither depends on it.

Motivation

Editor and diagram need to be visible side-by-side for edits to feel connected to their visual result - without a split view, users can't see the diagram update.

Proposed Implementation

No response

Definition of Done

• Implementation: Fully implemented according to the Serverless Workflow spec.
• Unit Tests: Comprehensive unit tests are included and passing.
• Integration Tests: Verified within the monorepo and target environments (Web/VS Code).
• Documentation: Updated README.md, ADRs, or official docs.
• Performance: No significant regression in editor responsiveness.
• Accessibility: UI changes comply with accessibility standards.