KeyVerificationPrompt

Key-verification prompt details surfaced via MeshEvent.KeyVerification.

Phase 1: marker interface only — emitted as a placeholder when the engine notices that encryption setup would prompt for confirmation, but no payload is exposed yet. Hosts should treat any non-null prompt as "show a generic verification UI" until the surface is filled in.

Phase 2+: this interface will gain at least:

public interface KeyVerificationPrompt {
    public val remoteNodeId: NodeId
    public val publicKeyFingerprint: String   // hex SHA-256, abbreviated for display
    public suspend fun confirm()              // accept; engine continues handshake
    public suspend fun reject()               // refuse; engine tears down with Protocol error
}

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).