ScopesPostVisibilityWithinDiscussion  

A post visibility scoper that can take a fast path when every post in the query belongs to one discussion the actor is already allowed to see.

The generic `view` scope must embed discussion-level access checks into the SQL as per-row subqueries, because the posts may span discussions. When the discussion is known and pre-authorized, those checks are constants: they can be evaluated once in PHP and the subqueries omitted, which matters on large discussions where the generic scope's cost grows with the post count. Implementations MUST apply the same post-level restrictions as their ordinary scoper; only discussion-level conditions may be lifted out. Scopers that don't implement this interface are simply applied as usual.