fix(primitives): eslint/tsconfig migration, asChild refactor, type fixes

- Migrate to eslint flat config + composite tsconfig.
- Complete the asChild→as="template" refactor (remove asChild prop + :as-child
  bindings across components, matching Primitive's slot model).
- Fix test type errors and source type-safety (useGraceArea hull/point math,
  FocusScope/util ref typing).

Note: ~53 vue-tsc errors remain (HTML attr/event passthrough typing on
transparent wrapper components + a couple of duplicate-export naming
collisions) — not gated by CI (build/lint/test green); pending a
component-attribute-typing design decision.

vue/primitives/src/scroll-area/ScrollAreaViewport.vue

@@ -0,0 +1,67 @@
+<script lang="ts">
+import type { PrimitiveProps } from '../primitive';
+
+export interface ScrollAreaViewportProps extends PrimitiveProps {
+  /** Inline `nonce` attribute applied to the injected style tag (CSP support). */
+  nonce?: string;
+}
+</script>
+
+<script setup lang="ts">
+import { onMounted, ref, watch } from 'vue';
+import { Primitive } from '../primitive';
+import { useForwardExpose } from '@robonen/vue';
+import { useScrollAreaRootContext } from './context';
+
+defineOptions({ inheritAttrs: false });
+
+defineProps<ScrollAreaViewportProps>();
+
+const ctx = useScrollAreaRootContext();
+const { forwardRef, currentElement } = useForwardExpose();
+
+const contentRef = ref<HTMLElement | null>(null);
+
+watch(currentElement, (el) => {
+  ctx.viewport.value = el ?? null;
+}, { immediate: true });
+
+watch(contentRef, (el) => {
+  ctx.content.value = el ?? null;
+}, { immediate: true });
+
+onMounted(() => {
+  ctx.viewport.value = currentElement.value ?? null;
+  ctx.content.value = contentRef.value ?? null;
+});
+</script>
+
+<template>
+  <!-- Hide native scrollbars while preserving native scrolling behaviour. -->
+  <component
+    :is="'style'"
+    :nonce="nonce"
+  >
+    [data-scroll-area-viewport]{scrollbar-width:none;-ms-overflow-style:none;-webkit-overflow-scrolling:touch;}[data-scroll-area-viewport]::-webkit-scrollbar{display:none;}
+  </component>
+
+  <Primitive
+    :ref="forwardRef"
+    :as="as"
+    :id="($attrs.id as string | undefined) ?? ctx.viewportId.value"
+    data-scroll-area-viewport=""
+    :style="{
+      overflowX: ctx.scrollbarXEnabled.value ? 'scroll' : 'hidden',
+      overflowY: ctx.scrollbarYEnabled.value ? 'scroll' : 'hidden',
+    }"
+    v-bind="$attrs"
+  >
+    <!-- A `min-width: fit-content` inner ensures horizontal content is measurable. -->
+    <div
+      :ref="(el: any) => { contentRef = el; }"
+      :style="{ minWidth: '100%', display: 'table' }"
+    >
+      <slot />
+    </div>
+  </Primitive>
+</template>