Move the model-route access modal off the deprecated allowed_users
policy/field onto the unified allowed_principals + principals surface,
persisting everything through a single /access POST.
- "specific users" radio now uses allowed_principals; a returned legacy
allowed_users value is normalized so existing routes still select it.
- derive the picker's selection and the full grant set from `principals`
in GET /access (fall back to legacy `items` if a backend doesn't
return principals yet).
- save as `principals`: the principal-based override sends its staged
set; the user picker maps its selection to USER-kind grants and
preserves any non-user grants from the snapshot (no longer sends
`users`).
- guard saving before the GET seeds principals (would wipe grants);
share the ALLOWED_PRINCIPALS_POLICY constant.
- AccessControlFormData: `users` optional, add `principals`.
