Read access to focus state during view/1 and update/2 callbacks.

Focus state itself lives in the runtime; this module exposes it via the process dictionary, set by the runtime immediately before invoking app callbacks. Don't try to call this from outside a Harlock callback — there's no global state to read.

Mutation (setting focus, advancing it manually) returns a Cmd to the runtime. v0.1 ships Cmd.focus/1; richer programmatic control arrives alongside the full Cmd executor.