KeyVerificationPrompt

Key-verification prompt details surfaced via MeshEvent.KeyVerification.

Marker interface; concrete verification methods will be added when PKI verification UI is implemented. Until then, any non-null prompt means the host should show a generic verification experience.

The shape will be ratified in a follow-up ADR before 1.0; consumers wiring this today should expect the interface to add abstract members (binary-incompatible for implementers, but a pure marker for collectors who only is-check it).