Template resolver: match page template ID to template component

[qantrepreneur]

What to build

Implement the template resolver that takes a page's template string and looks up the corresponding template component from the registered templates.

The resolver receives the template registry (populated at build time or from createApp) and the template ID from the page module. Returns the template component or throws a clear error if the template isn't found.

Acceptance criteria

• resolveTemplate(templateId, registry) returns the matching template component
• Throws descriptive error when template ID doesn't exist in registry
• Error message includes the invalid ID and lists available templates
• Unit tests for: valid lookup, missing template, empty registry

Blocked by

• Slot component with SlotContext and SlotProvider #4, definePage curried API with type inference #9