Show state machine errors in the State Machine view #155

Problem

The State Machine view shows error context only after the user clicks "View error on state machine" in the Verification view. If the user opens the State Machine view directly, the diagram does not show current state refinement errors, even though they belong to this machine.

Proposed behavior

• When the displayed machine has state refinement errors, show them in the State Machine view without requiring a click from the Verification view.
• Show a list of these errors beside the diagram. Each item should show the call, the receiver, Found, and Required, and link to its source location.
• Selecting an error applies the error focus and highlighting (Highlight state errors in the relevant @StateSet diagram #116, Focus the state machine on the rejected call and relevant states #117). Selecting none shows the general diagram.
• Match errors to the machine by class and state group, not by file alone. The error is usually reported in the client file, while the machine comes from the refinements file.
• Update the list when verification reruns. Remove fixed errors and keep the current selection if it still exists.
• Opening from "View error on state machine" preselects that error.

Acceptance criteria

• Opening the State Machine view for DefaultMutableTreeNode while the getFirstChild() error exists lists that error beside the diagram.
• Selecting the error focuses the diagram on it. Clearing the selection restores the general diagram.
• With no state errors, the view is unchanged.