fix(splitter): el asa recupera el velo del sistema

Firma 3, segundo componente, y la misma pareja de causas que el checkbox: el
asa lleva `archetype: 'trigger'`, así que `archetypes.css` le pinta el velo,
pero la regla base declaraba el ATAJO `background:` (0,1,0) y lo cancelaba
contra el `:where()` (0,0,0) del sistema. Encima había un hover propio con su
propio color.

  regla base    `background:` → `background-color:`
  regla hover   fuera entera; el velo la sustituye

Medido (812 valores, 7 estados): 2 diffs, ambos en el hover del asa —
`bg oklch(0.8853) → oklch(0.931)` y `img none → linear-gradient(rgb(0 0 0 /
0.08))`. Reposo, tallas, dragging, disabled y foco: idénticos. El estado
`dragging` conserva su acento (`--splitter-active-handle-bg`), que es un
hover CON VALENCIA y no le toca la capa neutra.

`handle-bg-hover` sale del contrato: sin consumidores.

  censo splitter 100 % · audit PASS · suite 434 (1 rojo conocido)

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

src/uix/eidos/components/splitter/splitter.css

@@ -49,7 +49,10 @@
 [data-splitter-resize-trigger] {
 	position: relative;
 	flex: 0 0 auto;
-	background: var(--splitter-handle-bg, var(--color-border-subtle));
+	/* LONGHAND: `background:` resets `background-image`, and this rule (0,1,0)
+	 * outweighs the `:where()` (0,0,0) of `archetypes.css` — the state-layer
+	 * veil would never paint on the handle. */
+	background-color: var(--splitter-handle-bg, var(--color-border-subtle));
 	transition: background var(--splitter-transition-duration, var(--duration-fast))
 		var(--splitter-transition-ease, var(--ease-default));
 }
@@ -64,10 +67,10 @@
 	cursor: row-resize;
 }
 
-[data-splitter-resize-trigger]:hover,
-[data-splitter-resize-trigger][data-state='hover'] {
-	background: var(--splitter-handle-bg-hover, var(--color-border-default));
-}
+/* The neutral hover is the system's veil (§38): the handle carries
+ * `archetype: 'trigger'`, so `archetypes.css` paints it. The bespoke rule that
+ * used to live here declared its own colour and, being the shorthand, was also
+ * what kept the veil from ever painting. */
 
 [data-splitter-resize-trigger]:focus-visible {
 	outline: var(--focus-ring-width) solid var(--focus-ring-color);