Yvatar implementation of the `AvatarProvider` seam.

This is the ONLY place that knows the Yvatar Developer Platform API — its
endpoints, its snake_case JSON, and its error codes. It maps the protocol's
provider-neutral `ctx`/persona to Yvatar's `external_provider` /
`external_user_id` mapping and back to the normalized result documented in
`game.protocols.avatar-provider`.

Endpoints (all under /api/v1/platform/users/:external-user-id/yvatar):
  PUT    .              upsert persona
  GET    .              fetch full persona
  POST   ./portrait     generate a pending portrait
  POST   ./portrait/accept  promote pending -> accepted
  GET    ./public       embed-safe public shape
  POST   ./content              start a video post (async — returns generating_video)
  GET    ./content              check the pending post's status (on-demand, not a poll loop)
  POST   ./content/regenerate   redraft the current pending post (async)
  POST   ./content/publish      publish the current draft post to a platform

The ./content* endpoints are video-only — Yvatar's Platform API does not
support image posts (the content studio's session-authenticated UI does, but
that surface is not exposed here). Video generation is asynchronous: POST
./content and POST ./content/regenerate return immediately with
status "generating_video"; GET ./content is how a caller checks whether it
finished, intended for a caller-side "refresh" action rather than a
background polling loop.

Auth: `Authorization: Bearer <yvtr_ token>` (developer-owned token).