The behaviour for focusable, stateful element kinds (input, select).

When a key reaches a focused component element, the runtime looks up the module for its tag and calls on_key/3 with the element's props (from the last rendered tree) and the component's ephemeral state (from Tuix.App private, e.g. an input's cursor offset). The component either:

• emits an event to the app ({:emit, event, state}) — components are controlled, so value changes are reported rather than applied
• updates only its ephemeral state ({:update, state}) — also used for consumed no-ops (e.g. backspace at the start of an input)
• ignores the key (:ignored) — it falls through to the app's Tuix.App.handle_event/2 with target set

Before painting, mark_props/2 computes props to merge into the focused element (e.g. the input's cursor) via Tuix.Focus.mark/3.