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>

docs/ui-visual-contract-0.8.json

@@ -29,7 +29,7 @@
     "packages/ui/components/Container.wrn": "be8fce140043eced8b78c8dd14ff85ace4ed2d91671251641ef83e0fe4da8f2a",
     "packages/ui/components/ContextMenu.wrn": "2e011e9cb1c09a3a2344ed3fa29dcc74331cd39f838535d8aee4b249dceef0ce",
     "packages/ui/components/CopyMarkup.wrn": "8d57a7d72e02c126f855a181fc36e1b969edb90f683dce6b6ffe546d49d2b29b",
-    "packages/ui/components/CustomScrollbar.wrn": "4c1f7758b9cd47e20ecf922403e12b5b15b8127a2744383c6bd280ee20e5fdd1",
+    "packages/ui/components/CustomScrollbar.wrn": "fedb2ab06cc2cf9499c130590945ca6971a3edfb60407b6658ef7756e4be9823",
     "packages/ui/components/DataMap.wrn": "65552d73ecd1a148427dbb10d611f9352ca36bd490f595ab1e1a47e173445ba1",
     "packages/ui/components/DataTable.wrn": "5a76e50e8559c724aaf01ec954958ac640c99758d44f1eef8e6eb1cb6dc4f862",
     "packages/ui/components/DatePicker.wrn": "18b07e59c27c2720bf960e9c04ad70d9e422ddcf0817fede74e71899811bb387",
@@ -53,7 +53,7 @@
     "packages/ui/components/InputGroup.wrn": "0e23ea541a60e893c9d9e6f95113911ee459468d8d6105bf03aeb55395359123",
     "packages/ui/components/InputNumber.wrn": "a0aa4f4566bf54ffbef54c99648eab05b5f9870be6ea07639a4ecf3930afb631",
     "packages/ui/components/Kbd.wrn": "d51fa614b36f971f885410785b481c2e6900b2416e6e7c8868dfda1ebd5f3b4c",
-    "packages/ui/components/LayoutSplitter.wrn": "dac5a80acb76bf58dfd25233c68190c67d1aa1b3b0d4250a0957ea40912cf722",
+    "packages/ui/components/LayoutSplitter.wrn": "a86dc54b74aed6b05e770cd1802960d6309bba42fe80e899e8aaa8b1b7aa88e3",
     "packages/ui/components/LegendIndicator.wrn": "6a7607b27a17196f899132eb952203a1073e4077365a94935f485e700cbac665",
     "packages/ui/components/Link.wrn": "935ac24e8ebd067860dc9709ea9121710315ad6e02ce95cd5e982ae4d5978a02",
     "packages/ui/components/List.wrn": "02f960a8e82fa049aac477bac9c1d9371394c86d9c47513138f61bffc79dc7d8",