* Copy FeedViewPost into FeedSliceItem * Explicitly construct feed slice items by copying known fields * Type rootItem as FeedViewPost for now Mergefeed logic relies on that. * Unify reason and __source for slice items * Move feedContext out of FeedSliceItem * Remove slice.isFlattenedReply * Remove unnused slice.ts * Inline slice.isFullThread * Refactor condition for clarity * Extract slice.includesThreadRoot * Encapsulate more usages of slice.rootItem into slice * Rename slice.rootItem so semi-private slice._feedPost * Move reason into slice * Simplify slice ctor argument * Reorder getters to reduce diff * Make feedContext a getter to reduce diff