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/ComboboxAnchor.vue

@@ -0,0 +1,33 @@
+<script lang="ts">
+import type { PopperAnchorProps } from '../popper';
+
+export interface ComboboxAnchorProps extends PopperAnchorProps {}
+</script>
+
+<script setup lang="ts">
+import { onBeforeUnmount, watchPostEffect } from 'vue';
+
+import { useForwardExpose } from '@robonen/vue';
+import { PopperAnchor } from '../popper';
+import { Primitive } from '../primitive';
+import { useComboboxRootContext } from './context';
+
+const props = defineProps<ComboboxAnchorProps>();
+
+const { forwardRef, currentElement } = useForwardExpose();
+const rootCtx = useComboboxRootContext();
+
+watchPostEffect(() => rootCtx.onParentChange(currentElement.value));
+onBeforeUnmount(() => rootCtx.onParentChange(undefined));
+</script>
+
+<template>
+  <PopperAnchor :reference="props.reference">
+    <Primitive
+      :ref="forwardRef"
+      :as="props.as ?? 'div'"
+    >
+      <slot />
+    </Primitive>
+  </PopperAnchor>
+</template>