Function reading_split 

pub fn reading_split(
    view: &ReviewView,
    group: &GroupView,
    fold: Fold,
) -> ReadingSplit

Split a group into its read and deferred halves.

• focus: everything is read.
• skim, folded: one exemplar per shape class; the rest is deferred, because verifying the exemplar verifies the shape.
• noise, folded: nothing is read.
• anything unfolded: everything is read.

Takes the PROJECTION, not a PlanIndex. Both renderers already hold one, and a PlanIndex borrows the document — so the TUI was building a fresh one on every keypress purely to reach the two class questions below.