feat(ui): build LayoutSplitter and CustomScrollbar for real

Both advertised behaviour they did not have. LayoutSplitter declared
resizeStart, resize and resizeEnd with no pointer handling whatsoever, so a
caller wired up @resize and received nothing, for ever, with no error, and its
props were columns, gap and maxWidth copied from a grid scaffold.
CustomScrollbar was the same shape with a scroll output.

The splitter now resizes. Dragging lives in the reactive runtime behind
data-wrn-splitter, because a pointermove fires far too often to route through
a client function and a state write made in that callback is dropped; the
resolved size is held on the container as a --wrn-split custom property and
the component grids from it. The handle is a real separator: arrow keys step
it, Home and End go to the bounds rather than to nothing, and it carries
aria-valuenow, aria-valuemin and aria-valuemax. minSize fixes both bounds so
neither pane can be dragged away and left unrecoverable.

CustomScrollbar is CSS rather than script -- scrollbar-width and
scrollbar-color with webkit rules for the engines that still need them -- and
its fake scroll output is removed rather than left unimplemented, since a
caller can listen for a plain scroll event.

The test harness needed a fix too: mount did not bind the window CustomEvent,
so the runtime built events from the host global and happy-dom listeners never
matched them, which made anything dispatched look silently lost.

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

examples/component-showcase/app/styles/global.css

@@ -2960,3 +2960,31 @@ body {
   font-size: 0.86rem;
   line-height: 1.6;
 }
+
+/*
+ * CustomScrollbar demo content. A scrolling region shows nothing at all unless
+ * its content overflows, so the demo has to supply enough of it.
+ */
+.showcase-scroll-demo {
+  display: grid;
+  gap: 0.5rem;
+  padding: 0.25rem;
+}
+
+.showcase-scroll-demo p {
+  margin: 0;
+  color: var(--wire-color-text-muted);
+  font-size: 0.85rem;
+}
+
+.showcase-scroll-demo--wide {
+  grid-auto-flow: column;
+  grid-auto-columns: 9rem;
+}
+
+.showcase-scroll-demo--wide span {
+  padding: 0.75rem;
+  border: 1px solid var(--wire-color-border);
+  border-radius: var(--wire-radius-sm);
+  font-size: 0.85rem;
+}