safe-ui: say why the airlock sequence lives in Rust

The module doc claimed full Slint would express the sequence in .slint,
which nobody would: an airlock state machine is application logic, and it
belongs in Rust in any Slint program. The claim also undercuts what this
example is for, where the screen deciding nothing is the point rather
than a limit of the subset.

The README carried the same claim, naming a Timer and a global.