Custom signing runtime

@seams/sdk/runtime is for hosts that supply the platform around the signing engine. It is a lower-level integration than SeamsWeb.

The entrypoint exports createSigningRuntime, createSigningRuntimeStatePorts, and types for the runtime configuration, services, state ports, UI dependencies, and warm-session UI ports.

Platform responsibilities are explicit through ports for:

• authentication;
• time and randomness;
• HTTP transport;
• durable records and secure secrets;
• signer cryptography;
• runtime-specific UI.

Choose one RuntimePortsKind and provide the complete matching RuntimePorts branch. Validate raw host data before constructing those ports. A custom runtime also owns secure storage, cancellation, lifecycle restoration, and user-presence semantics, so use the browser SDK unless those responsibilities are deliberate.

Read architecture and origin and iframe boundaries first.