Explicit initial transition: reject a contradictory initial=True child and draw it in diagrams #711

Why

Follow-up from #694: with an explicit initial transition on a compound state, a child also marked initial=True is silently overridden, and diagrams still draw the first child as the initial state. Contradictory definitions should be rejected with InvalidDefinition.

Acceptance

Tests for the rejection and the diagram output.

Refs #587, #692