Add the SDK surface for plugins to deposit images into the CMS media library.
MediaDeposit / MediaResult types and a Media interface on CoreServices for
runtime deposits; EnsureMedia on the Provisioner interface for idempotent
seed-time deposits under a plugin-chosen, template-referable UUID. The CMS
implements both against one internal depositor; seeded templates reference
the chosen UUID directly via {% img "<id>" %}.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>