Add Stage execution and cross-process manual continuation

[taras]

Superseded

The Stage construct and stage boundaries were rejected as architecture: the root document is the workflow, and a durable workflow may continue through multiple document executions without inventing subdivisions between them.

The valid requirements from this issue now belong to #291:

• durable workflow lookup;
• xmd workflow run and xmd workflow continue;
• restoration of declared named values;
• cross-process continuation without transcript copying;
• durable outcomes and stop reasons; and
• no implicit approval when user input is required.

This issue is retained as closed history rather than deleted.