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/combobox/ComboboxSeparator.vue

@@ -0,0 +1,26 @@
+<script lang="ts">
+import type { PrimitiveProps } from '../primitive';
+
+export interface ComboboxSeparatorProps extends PrimitiveProps {}
+</script>
+
+<script setup lang="ts">
+import { useForwardExpose } from '@robonen/vue';
+
+import { Primitive } from '../primitive';
+
+const { as = 'div' } = defineProps<ComboboxSeparatorProps>();
+const { forwardRef } = useForwardExpose();
+</script>
+
+<template>
+  <Primitive
+    :ref="forwardRef"
+    :as="as"
+    role="separator"
+    aria-orientation="horizontal"
+    aria-hidden="true"
+  >
+    <slot />
+  </Primitive>
+</template>