History-sharing-on-invite: key bundles from unknown senders are ignored when "Exclude insecure devices" is set

Suppose Alice and Bob do not currently share a room.

* Alice invites Bob to an encrypted room, and shares history.
* Bob receives the to-device message, including the `sender_device_keys`, but he does not yet have a copy of Alice's identity key.
* He is therefore unable to verify the cross-signature on the device in `sender_device_keys`, and ignores the to-device message

No files, tests, or entry points are named. Start by tracing handling of the history-sharing to-device message, especially sender_device_keys and cross-signature verification when the sender's identity key is unknown. Done should mean the described invite and shared-history flow no longer ignores the key bundle solely because the sender cannot yet be verified.

Written by the indexing model from the issue text.