Stepper

Accessible name of the navigation landmark. Defaults to `copy.navLabel`, which an empty string also falls back to (`label || copy.navLabel`), so the landmark is never unnamed.

The steps in order. `status` is derived from `current` when omitted: before it complete, the step it names current, after it upcoming. An explicit `status` sets only the indicator, its colours and the status word; position (not status) decides the selected state, navigability, the connector colour and the compact reveal.

The id of the current step. The step whose id matches is the selected one (`aria-current="step"` / selected state) and the one compact reveals, whatever its `status`. When no id matches, nothing is selected, every step without an explicit status is upcoming, no step is navigable under `completed` (all still are under `all`), the count reads "Step 1 of m", compact reveals the first step's label (so a compact stepper is never label-less), and development builds log a warning — a developer-only English message, not copy, worded identically on every platform: `Stepper: current "<id>" matches no step id; nothing is selected.` The warning needs a non-empty `current`: an empty one (the Lit property's initial `''`) is treated as not yet set and does not warn.

Vertical shows descriptions under each label and suits a side column; horizontal does not render descriptions at all (not clipped, and no aria-describedby) and collapses to `compact` below the prose width.

Which steps can be activated: none (display only), completed steps (the usual — you can go back, not skip ahead), or all (a settings-style flow where order does not matter). "Completed" means visited — any step before the current one by position, including one marked `error`, which is exactly the step a user most needs to return to. The control is the component's own native button, not the Button component, whose single-label API cannot hold an indicator, a label and a description.

Show only the current step's label and "Step 2 of 5"; the indicators stay. Horizontal only, set by hand or automatically below the prose width — a vertical stepper has the room, so the prop does nothing there, including the `count`, which a vertical stepper never renders. On web and Lit the labels of the other steps, with their status words, are visually clipped (the visually-hidden technique), not removed, so they stay reachable by a screen reader; on React Native they are not rendered, because each step's accessibilityLabel already carries the label and status. "Step n of m" (`copy.stepOf`) is the `count` part: one muted Text after the list, not in any step and not replacing a description. On web and Lit, where automatic compact is a container query, it is always rendered and `display: none` outside the query (applied through Text's layout-only className); on React Native it is rendered only while compact is in effect. The count follows the list, `stepGap` after it — as the root's own flex/column gap, never a margin, so the gap collapses with the count when it is hidden instead of leaving dead space under every non-compact stepper.

Fired when a navigable step is chosen, with its id. The container changes `current`; the stepper never changes it itself.