Add bus fanout plane termination props

[seveibar]

Summary

• add a typed fanoutTermination property to <bus>
• support boundary and layer-specific plane termination
• permit source-only singleton buses when they terminate on a plane
• add fanoutRoutingLayers to keep signal escapes off reserved plane layers
• regenerate public prop documentation

Why

The fanout solver can already escape power and ground pads directly to an internal plane, but JSX bus props cannot express that termination. This exposes the solver capability without changing existing boundary-routed bus behavior.

Validation

• bun test — 396 passed
• bun run typecheck
• bun run format:check
• bun run build