v0.3.0

Findings of the cska-one consumer pilot:

• siteName falls back to providerName before the host (hostAsSiteName):
  a card known only by its oEmbed provider is no longer signed by its domain.
• A pinning client's own body limit (rasuvaeff/pinned-http-client
ResponseTooLarge) is TooLarge, not ClientRefused — the two limits no
  longer need the "+1 byte" dance to tell a big body from a refusal.
• fetchMedia(): a 2xx answer with an empty body is the new
  FetchFailureReason::EmptyBody / PreviewFailureReason::EmptyBody
  (permanent) instead of an empty success.
• An unresolvable host from pinned-http-client 0.3 (ResolutionFailed, a
  NetworkExceptionInterface) is Network — transient; dev/suggest bumped
  to ^0.3.
• Requires rasuvaeff/schema ^0.6 (internal answer schemas; diagnostics now
  quote non-plain keys in paths).