ui: the invert disc's half is actually filled — and the demos match

Lucide's Contrast leaves its half-moon unfilled (fill=none default) -
the owner's catch. Hand-rolled InvertDiscIcon: same geometry, the half
carries fill=currentColor, so the filled side is filled. The demo cards
get the identical disc + 'Invert colors' tooltip on their invert
toggles, matching the figures.

components/artifacts/invert-disc.tsx

@@ -0,0 +1,23 @@
+/** The invert disc (owner round, 2026-10-07): the half-filled circle that
+ *  IS the invert toggle's sliding disc. Hand-rolled because lucide's
+ *  Contrast leaves its half-moon unfilled (fill="none" by default) — the
+ *  owner's catch: the filled side must be ACTUALLY filled. Same geometry
+ *  as lucide's (circle r10 + left half-moon), the half carries
+ *  fill="currentColor". */
+export function InvertDiscIcon({ className }: { className?: string }) {
+	return (
+		<svg
+			viewBox="0 0 24 24"
+			fill="none"
+			stroke="currentColor"
+			strokeWidth="2"
+			strokeLinecap="round"
+			strokeLinejoin="round"
+			className={className}
+			aria-hidden
+		>
+			<circle cx="12" cy="12" r="10" />
+			<path d="M12 18a6 6 0 0 0 0-12v12z" fill="currentColor" stroke="none" />
+		</svg>
+	);
+}