screen-shell-content-split

Split one screen into a shell that owns every cross-look concern and a content layer that only renders, connected by two holders — a state snapshot and an actions bag — so adding a second look costs one branch instead of a parameter-list edit; covers why the holders are stable-but-not-immutable, what must stay in the shell, and what must not. Use when a screen has grown a second visual style, when a style switch…