Separate portal code from shared @vuu-ui/core entry

[heswell]

Summary

• keep the shared @vuu-ui/core root limited to authentication and data-context APIs
• expose portal components through the non-shared @vuu-ui/core/portal subpath
• preserve self-imports from portal code to the shared root so authentication contexts remain singleton
• publish both package entry points and update portal consumers

Validation

• built @vuu-ui/core and verified the published export map
• built the vuu-portal host
• built the feature-simple-div federated remote
• confirmed MF manifests share only @vuu-ui/core
• passed all 10 core authentication tests