Fix Mermaid composite activity labels

[SandroMaglione]

Summary

• Keep activities owned by composite or parallel states in the state's single Mermaid group label.
• Preserve the existing description syntax for activities owned by leaf states.
• Cover composite child-machine activities with a focused regression assertion.

The renderer previously emitted both a labelled composite declaration and a second state_id: description. Mermaid rejects that representation with Group nodes can only have label.

Changeset

• Added or updated for a library or package-metadata change
• Not required because this PR does not change src/ or package.json

Validation

• pnpm check
• Relevant example checks, when examples changed (not applicable)
• Automated type-performance measurement passed or was not required
• Automated runtime- and memory-performance measurement passed or was not required