WAIT: Save logged_out_only answers for signed-in registrants

[maebeale]

What is the goal of this PR and why is this important?

• When a signed-in registrant has identity/contact data on file, those logged_out_only fields are hidden from the registration form, so their answers were never recorded on the submission — leaving an incomplete snapshot that looked like the registrant skipped questions.
• This records that on-file data as form answers so each registration is a complete snapshot, and makes the field-hiding rule consistent with the bulk-payment form.

How did you approach the change?

• Added a PersonKnownFields PORO that maps a person's on-file identity/contact data to the registration form's logged_out_only field identifiers, and PublicRegistration now backfills those values onto the submission for signed-in registrants (skipping confirm_email, blanks, and anything the registrant actually submitted).
• Changed PublicRegistrationsController to hide all logged_out_only fields for signed-in users (matching the bulk-payment rule) instead of only the ones we already had answers for, with a temporary exception keeping the Organization Information (agency_*) fields visible until they can be backfilled from affiliations.

UI Testing Checklist

• As a signed-in registrant, the identity/contact fields you already have on file are hidden, but the Organization Information section still shows.
• After submitting, the saved registration shows answers for those hidden fields populated from your profile data.
• As a logged-out registrant, every field still renders as before.

Anything else to add?

• The Organization carve-out is intentionally temporary; once org data is backfilled from person.affiliations.active, the agency_* exception in hide_logged_out_only_fields comes out.
• The scholarship-form rendering still applies no logged_out_only filter — left untouched in this PR.