spec rules for selecting a room avatar if there is no explicit one

i.e. the avatar equivalent of https://matrix.org/docs/spec/client_server/r0.2.0.html#calculating-the-display-name-for-a-room

Read the referenced Matrix client-server specification section on calculating a room's display name first. Define the equivalent rules for selecting a room avatar when no explicit avatar is present, then add the agreed behavior to the appropriate specification section; completion requires a clear, reviewed normative rule.

Written by the indexing model from the issue text.