[Feature]: As a process developer, I want to be warned about process models that can never be reached

[emaarco]

Summary

As a process developer, I want to be warned about process models that no call activity references and that have no start event, so that I can spot dead/orphaned models in my fileset.

Motivation

Unreachable processes are usually copy-paste leftovers or renamed calledElement references and produce needless generated code.

Proposed Solution

• New opt-in CrossModelValidationRule (unreachable-process, WARN).
• A process is unreachable when: (a) no callActivity.getValue() across all models points to its processId and (b) it has no start event (BpmnNodeType.Event(EventShape.START_EVENT, …)).
• No domain change — EventShape.START_EVENT is available.
• Document the false-positive risk (top-level processes started via API); hence WARN + opt-in.

Acceptance Criteria

• Warns only on "not called and no start event"
• Process with a start event or an incoming call activity → no warning
• Opt-in, core unit test + integration test, docs with false-positive note