Issue #3594391: Add a Tugboat preview configuration (single-site, local-mode demo)

Adds .tugboat/config.yml + .tugboat/setup.php (mirroring the audit_trail setup) for a single-site, local-mode preview.

Enables: pdv, pdv_client, pdv_test_console, pdv_webform (plus key, consumers, webform, admin_toolbar).

Seeds: a 256-bit Master KEK (Key, config provider) pointed at by pdv.settings:master_key_id; a demo owner (owner / owner) with a couple of documents; a demo consumer for the grant / trust / consent flows; a guide front page; daily rebuild.

Out of scope: cross-site (pdv_server_api over HTTP, needs a second site) and the audit-trail bridge (no chain in its config/install; a misconfigured audit would break vault writes).

Note: Tugboat must also be enabled for the project in its Drupal.org settings for previews to run.