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/aspect-ratio/AspectRatio.vue

@@ -0,0 +1,41 @@
+<script lang="ts">
+import type { PrimitiveProps } from '../primitive';
+
+export interface AspectRatioProps extends PrimitiveProps {
+  /**
+   * Desired width-to-height ratio (e.g. `16 / 9`, `1`, `4 / 3`).
+   * @default 1
+   */
+  ratio?: number;
+}
+</script>
+
+<script setup lang="ts">
+import { Primitive } from '../primitive';
+import { useForwardExpose } from '@robonen/vue';
+
+useForwardExpose();
+
+const { ratio = 1, as = 'div' } = defineProps<AspectRatioProps>();
+
+const wrapperStyle = {
+  position: 'relative' as const,
+  width: '100%',
+  paddingBottom: `${(1 / ratio) * 100}%`,
+};
+
+// Hoisted constant — the inner style never depends on props, so a single
+// module-level object is reused across all instances.
+const INNER_STYLE = {
+  position: 'absolute' as const,
+  inset: 0,
+};
+</script>
+
+<template>
+  <div :style="wrapperStyle" data-aspect-ratio-wrapper>
+    <Primitive :as="as" :style="INNER_STYLE" :data-aspect-ratio="true">
+      <slot />
+    </Primitive>
+  </div>
+</template>