Register publickey-credentials-remote-client-data-json feature - #586

Adds the publickey-credentials-remote-client-data-json policy-controlled feature to the registry (features.md).

What this gates

The feature controls the remoteClientDataJSON WebAuthn client extension, which lets an authorized remote-desktop web client forward a verbatim clientDataJSON to the local authenticator (rather than having the local user agent reconstruct it).

Where it is normatively defined

The feature is defined by the Web Authentication API in its Permissions Policy integration section, consistent with the registry convention that "the exact definition of the behaviour controlled by each feature belongs in the spec which defines the feature." The registry row therefore points at the same WebAuthn section as publickey-credentials-get.

Default allowlist

Unlike publickey-credentials-create and publickey-credentials-get (default allowlist self), this feature has a default allowlist of none. It is disabled for all origins, including the top-level document, unless explicitly delegated. The WebAuthn spec also designates it a powerful feature with default permission state "denied".

Note that default allowlist of none is currently not normative, but exists as a proposed change in #515

Placement

Added to the Standardized Features table (the normative definition is already integrated into the WebAuthn editor's draft). Browser support is marked ChromeOpen pending the implementation landing as experimental.