Content: a token field type, generated on create and never writable

Found by simulating a Claude Code build of Color Me Fun 2026 through the barako CLI (epic #801) on 14 September 2026; the event registration app for the Rotary Club of Koronadal is the acceptance test. Simulation page: https://claude.ai/code/artifact/05afda1a-6461-431d-aff0-50b30d3b0778

References are sequential and printed; a claim stub must be unguessable (CONTRACTS.md rule 3). No field type generates a secret value on create.

## Change

A `token` field type: generated on create from a cryptographic random source, a configurable length, never writable through the API, never publicly delivered unless a lookup (see the public lookup issue) or QR rendering explicitly allows it, and unique within the type.

## Done when

- Creating an entry fills the token; an update that tries to set it is refused.
- Two entries never share a token, and public delivery of the type omits it.

Start with CONTRACTS.md rule 3 and the field-type, create/update, and public-delivery entry points used by the barako CLI; the issue names no concrete files or tests. Trace the public lookup and QR-rendering paths, using the Color Me Fun 2026 registration app as the acceptance scenario, and verify generated values, rejected updates, uniqueness, and omitted public delivery.

Written by the indexing model from the issue text.