feat(ui): build the Stepper component

Replaces a scaffold that rendered bare anchors with ordered steps: complete,
current and upcoming status derived from the active index, horizontal or
vertical, optional icons, and indexed named slots (step-0, step-1, ...) for
authoring a step body by hand.

Also tightens the roving contract from the previous commit. A template writes
data-wrn-roving="" or data-wrn-roving-item="false" to mean not this time, but
a bare [attr] selector matches either, so a read-only stepper would still have
taken arrow-key focus. The container now requires a named axis, while a bare
item marker still counts as opted in.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/csr/src/reactive-runtime.ts

@@ -3069,11 +3069,35 @@ export const REACTIVE_RUNTIME = String.raw`
   var ROVING_SELECTOR = "[data-wrn-roving]";
   var ROVING_ITEM_SELECTOR = "[data-wrn-roving-item]";
 
+  /*
+   * These attributes are written by templates, so they arrive stringified:
+   * data-wrn-roving="" or data-wrn-roving-item="false" is how a component
+   * says "not this time". A bare [attr] selector matches either, so the value
+   * has to be checked -- otherwise a stepper with clickable=false still takes
+   * arrow-key focus.
+   */
+  // A bare data-wrn-roving-item means yes; only an explicit "false" opts out.
+  function rovingItemOff(value) {
+    return value === null || value === "false";
+  }
+
+  /*
+   * The container is stricter: it must name an axis. An empty value is what a
+   * template emits for {cond ? "horizontal" : ""}, so empty means off rather
+   * than defaulting to horizontal.
+   */
+  function rovingOrientation(container) {
+    var value = container.getAttribute("data-wrn-roving");
+    if (value === null || value === "" || value === "false") return "";
+    return value;
+  }
+
   function rovingItems(container) {
     var found = [];
     var candidates = container.querySelectorAll(ROVING_ITEM_SELECTOR);
     for (var index = 0; index < candidates.length; index += 1) {
       var candidate = candidates[index];
+      if (rovingItemOff(candidate.getAttribute("data-wrn-roving-item"))) continue;
       // A nested group owns its own items; do not steal them.
       if (candidate.closest(ROVING_SELECTOR) !== container) continue;
       if (candidate.hasAttribute("disabled")) continue;
@@ -3107,6 +3131,7 @@ export const REACTIVE_RUNTIME = String.raw`
   }
 
   function syncRovingGroup(container) {
+    if (!rovingOrientation(container)) return;
     var items = rovingItems(container);
     if (!items.length) return;
     applyRovingTabindex(items, rovingActiveIndex(items));
@@ -3129,7 +3154,8 @@ export const REACTIVE_RUNTIME = String.raw`
     var index = items.indexOf(item);
     if (index === -1) return;
 
-    var orientation = container.getAttribute("data-wrn-roving") || "horizontal";
+    var orientation = rovingOrientation(container);
+    if (!orientation) return;
     var horizontal = orientation === "horizontal" || orientation === "both";
     var vertical = orientation === "vertical" || orientation === "both";
     var key = event.key;