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>

packages/ui/COMPONENTS.md

@@ -633,9 +633,9 @@ Constrain and align page content with responsive gutters and compact, wide, or f
 Theme-aware, responsive custom scrollbar component.
 
 - Mount: `data-component="CustomScrollbar"`
-- Props: `size: string = "default"`, `color: string = "primary"`, `columns: number = 2`, `gap: string = "md"`, `maxWidth: string = "xl"`, `class: string = ""`
+- Props: `color: string = "primary"`, `size: string = "default"`, `axis: string = "vertical"`, `thickness: number = 8`, `maxHeight: string = "20rem"`, `radius: string = "999px"`, `class: string = ""`
 - Slots: `default`
-- Outputs: `scroll({ sourceEvent?: Event; value: string | number | boolean | null | object; [key: string]: string | number | boolean | null | object } | string | number | boolean | null | object[])`
+- Outputs: None
 
 ### Divider
 
@@ -696,9 +696,9 @@ Theme-aware, responsive kbd component.
 Theme-aware, responsive layout splitter component.
 
 - Mount: `data-component="LayoutSplitter"`
-- Props: `size: string = "default"`, `color: string = "primary"`, `columns: number = 2`, `gap: string = "md"`, `maxWidth: string = "xl"`, `class: string = ""`
-- Slots: `default`
-- Outputs: `resizeStart({ sourceEvent?: Event; value: string | number | boolean | null | object; [key: string]: string | number | boolean | null | object } | string | number | boolean | null | object[])`, `resize({ sourceEvent?: Event; value: string | number | boolean | null | object; [key: string]: string | number | boolean | null | object } | string | number | boolean | null | object[])`, `resizeEnd({ sourceEvent?: Event; value: string | number | boolean | null | object; [key: string]: string | number | boolean | null | object } | string | number | boolean | null | object[])`
+- Props: `color: string = "primary"`, `size: number = 50`, `orientation: string = "horizontal"`, `minSize: number = 15`, `step: number = 5`, `label: string = "Resize panels"`, `class: string = ""`
+- Slots: `start`, `end`, `default`
+- Outputs: `resize({ size: number })`
 
 ### Link