Spec 0008 D5, rung 1 of the ladder in D10. NewProviderFromClient takes a
*giteasdk.Client the caller already holds and builds a provider on it. Both
registrations — Gitea and Codeberg — reach it.

The rung TRANSFERS THE CREDENTIAL OBLIGATION, exactly as WithHTTPClient (D4)
transfers the redirect-policy obligation. Settings.Credential must be nil, and
supplying one is ErrCredentialWithClient rather than a silent preference.

ENDPOINT.HOST IS REQUIRED HERE, which is an asymmetry with the other two SDK
adapters and worth stating plainly. go-github and go-gitlab both expose an
accessor for the instance they address, so those adapters read it back off the
injected client. gitea.dev/sdk keeps its url field unexported and offers no
accessor — checked at v1.2.0, where every exported method on Client is a typed
API call or a setter. The host is what the Sites capability derives a published
site's domain from, so an absent one is ErrHostRequiredWithClient rather than a
default that would point at the wrong instance.

ASSET DOWNLOADS ARE ANONYMOUS AT THIS RUNG. A Gitea asset is fetched from
browser_download_url, which is not an API call: the SDK never makes it and offers
no method that would — GetReleaseAttachment returns metadata, not bytes. This
provider fetches it directly and would normally attach its own Authorization
header, gated by forge.HostTrusted; it has no credential to attach here and the
SDK exposes no way to read one back.

A public asset downloads and a private one does not. A caller who needs
authenticated downloads uses a lower rung, where this module still owns the
credential.

Every guard is falsified against the mutant that removes it, including inventing
a token, which fails TestFromClientDownloadsAnonymously.