Main MVU model update function. Wraps the dispatch so that a library sheet opened for viewing is held at the state it loaded with - see ModelHelpers.pinDrawBlock. It has to be done here rather than inside Sheet.update: the three draw block update functions nest, so gating there would catch every message, but a handful of places edit model.Sheet directly with Optic.map and reach none of them - the separate/reroute wires actions in Renderer.fs, KeyBindings.fs and TopMenuView.fs, two context menu items in UpdateHelpers.fs, the simulator's "save input values as default", and the memory refresh that runs on every message. All of those pass through here.

Main MVU model update function, before a read-only sheet is held at its pinned state. Call `update`, not this.