OptionalhostOptional canvas-local host dimensions (CSS px). When wired,
viewportZoomAction's keyboard branches (Cmd+= / Cmd+-) anchor at the
host center instead of the top-left origin. Null when the host isn't
measurable (unmounted).
OptionalrecenterOptional recenter callback. When wired, viewportZoomAction's
reset branch (Cmd-0) calls this instead of resetting to identity —
letting consumers re-fit the page (or other reference bounds) into
the workspace. Receives no args; the consumer reads its own bounds
setView(...).
Minimal view API the action layer consumes. May be refined later.