Description

Every InsertRune→followCursor→buildVisual rescans+rewraps all lines; MoveUp/MoveDown call buildVisual then followCursor calls it again (2 full scans/vertical keystroke). Fine for notes, costly for long docs in the writing canvas. Fix: thread one []vrow through move→followCursorWith(rows); longer term cache the visual model, invalidate on edit/resize.

Acceptance Criteria

• #1 at most one buildVisual per keystroke