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/pages/index.wrn

@@ -17,11 +17,11 @@ page ComponentShowcase {
       <section class="home-product-grid"><article><span class="icon-[lucide--component] size-7"></span><strong>108 components</strong><p>Accessible primitives for every product surface.</p><a href="/base">Browse components →</a></article><article><span class="icon-[lucide--layout-template] size-7"></span><strong>Ready-made blocks</strong><p>Composable sections assembled from WRNexus UI.</p><a href="/block-library">Explore blocks →</a></article><article><span class="icon-[lucide--panels-top-left] size-7"></span><strong>Page templates</strong><p>Complete responsive pages ready to customize.</p><a href="/templates">View templates →</a></article></section>
       <section class="showcase-stats">
         <div class="showcase-stat"><strong>108</strong><span>Unique components</span></div>
-        <div class="showcase-stat"><strong>421</strong><span>Live configurations</span></div>
+        <div class="showcase-stat"><strong>419</strong><span>Live configurations</span></div>
         <div class="showcase-stat"><strong>0</strong><span>Duplicate implementations</span></div>
         <div class="showcase-stat"><strong>11</strong><span>Focused categories</span></div>
       </section>
-      <section class="home-categories"><div class="home-section-heading"><span>Explore the system</span><h2>Everything your product needs</h2></div><div class="home-category-grid"><a class="home-category home-category--1" href="/advanced-forms"><span class="home-category-index">01</span><div><strong>Advanced Forms</strong><p>8 components · 112 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--2" href="/base"><span class="home-category-index">02</span><div><strong>Base</strong><p>27 components · 86 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--3" href="/core"><span class="home-category-index">03</span><div><strong>Core</strong><p>5 components · 13 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--4" href="/data"><span class="home-category-index">04</span><div><strong>Data</strong><p>3 components · 9 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--1" href="/forms"><span class="home-category-index">05</span><div><strong>Forms</strong><p>12 components · 36 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--2" href="/integrations"><span class="home-category-index">06</span><div><strong>Integrations</strong><p>11 components · 33 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--3" href="/layout"><span class="home-category-index">07</span><div><strong>Layout</strong><p>15 components · 45 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--4" href="/marketing"><span class="home-category-index">08</span><div><strong>Marketing</strong><p>10 components · 30 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--1" href="/navigation"><span class="home-category-index">09</span><div><strong>Navigation</strong><p>10 components · 30 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--2" href="/overlays"><span class="home-category-index">010</span><div><strong>Overlays</strong><p>6 components · 18 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--3" href="/tables"><span class="home-category-index">011</span><div><strong>Tables</strong><p>1 components · 9 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a></div></section>
+      <section class="home-categories"><div class="home-section-heading"><span>Explore the system</span><h2>Everything your product needs</h2></div><div class="home-category-grid"><a class="home-category home-category--1" href="/advanced-forms"><span class="home-category-index">01</span><div><strong>Advanced Forms</strong><p>8 components · 112 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--2" href="/base"><span class="home-category-index">02</span><div><strong>Base</strong><p>27 components · 86 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--3" href="/core"><span class="home-category-index">03</span><div><strong>Core</strong><p>5 components · 13 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--4" href="/data"><span class="home-category-index">04</span><div><strong>Data</strong><p>3 components · 9 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--1" href="/forms"><span class="home-category-index">05</span><div><strong>Forms</strong><p>12 components · 36 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--2" href="/integrations"><span class="home-category-index">06</span><div><strong>Integrations</strong><p>11 components · 33 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--3" href="/layout"><span class="home-category-index">07</span><div><strong>Layout</strong><p>15 components · 43 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--4" href="/marketing"><span class="home-category-index">08</span><div><strong>Marketing</strong><p>10 components · 30 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--1" href="/navigation"><span class="home-category-index">09</span><div><strong>Navigation</strong><p>10 components · 30 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--2" href="/overlays"><span class="home-category-index">010</span><div><strong>Overlays</strong><p>6 components · 18 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--3" href="/tables"><span class="home-category-index">011</span><div><strong>Tables</strong><p>1 components · 9 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a></div></section>
     </div>
   }
 }