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/ComboboxPortal.vue

@@ -0,0 +1,17 @@
+<script lang="ts">
+import type { PortalProps } from '../teleport';
+
+export interface ComboboxPortalProps extends PortalProps {}
+</script>
+
+<script setup lang="ts">
+import { Portal } from '../teleport';
+
+const { to, defer, disabled } = defineProps<ComboboxPortalProps>();
+</script>
+
+<template>
+  <Portal :to="to" :defer="defer" :disabled="disabled">
+    <slot />
+  </Portal>
+</template>