entryBuilder property

Optional builder for fully custom timed-event widgets (#78). When non-null the planner layers a widget overlay over the events canvas and calls this for every on-screen event, sizing/positioning the returned widget at the event's current on-screen rect so it tracks scroll, zoom and drag in lockstep with the grid; the canvas then skips painting the default event bodies (the grid lines and accessibility semantics stay intact).

The overlay is visual-only — wrapped in IgnorePointer/ExcludeSemantics — so every gesture and accessibility action still falls through to the built-in recognizers and fires the usual config.onEntry* callbacks. When null (the default) events render exactly as before, painted on the canvas.