feat(ui): add decorative style primitives

src/lib/ui/stories/ScanlineField.stories.svelte

@@ -0,0 +1,20 @@
+<script module lang="ts">
+  import { defineMeta } from "@storybook/addon-svelte-csf";
+  import ScanlineField from "../components/ScanlineField.svelte";
+  import Panel from "../components/Panel.svelte";
+
+  const { Story } = defineMeta({
+    title: "Style Primitives/ScanlineField",
+    component: ScanlineField,
+    tags: ["autodocs"],
+  });
+</script>
+
+<Story name="Soft Neutral" args={{ intensity: "soft", tone: "neutral", size: "md" }} />
+<Story name="Medium Accent" args={{ intensity: "medium", tone: "accent", size: "md" }} />
+<Story name="Hard Warning" args={{ intensity: "hard", tone: "warning", size: "lg" }} />
+<Story name="Panel Backdrop" asChild>
+  <Panel title="Signal Surface">
+    <ScanlineField intensity="hard" tone="accent" />
+  </Panel>
+</Story>