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/showcase-manifest.json

@@ -2,7 +2,7 @@
   "generatedFrom": "packages/ui/component-reference.json",
   "componentCount": 108,
   "categoryCount": 11,
-  "totalDemoCount": 421,
+  "totalDemoCount": 419,
   "components": [
     {
       "name": "Accordion",
@@ -393,11 +393,11 @@
       "slug": "custom-scrollbar",
       "category": "layout",
       "purpose": "Theme-aware, responsive custom scrollbar component.",
-      "demoCount": 3,
-      "propCount": 6,
+      "demoCount": 2,
+      "propCount": 7,
       "slots": ["default"],
-      "events": ["scroll"],
-      "profiled": false
+      "events": [],
+      "profiled": true
     },
     {
       "name": "DataMap",
@@ -690,11 +690,11 @@
       "slug": "layout-splitter",
       "category": "layout",
       "purpose": "Theme-aware, responsive layout splitter component.",
-      "demoCount": 3,
-      "propCount": 6,
-      "slots": ["default"],
-      "events": ["resizeStart", "resize", "resizeEnd"],
-      "profiled": false
+      "demoCount": 2,
+      "propCount": 7,
+      "slots": ["start", "end", "default"],
+      "events": ["resize"],
+      "profiled": true
     },
     {
       "name": "LegendIndicator",