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/date-picker/DatePickerAnchor.vue

@@ -0,0 +1,28 @@
+<script lang="ts">
+import type { PopperAnchorProps } from '../popper';
+
+export interface DatePickerAnchorProps extends PopperAnchorProps {}
+</script>
+
+<script setup lang="ts">
+import { onBeforeMount, onUnmounted } from 'vue';
+import { PopperAnchor } from '../popper';
+import { useDatePickerRootContext } from './context';
+
+const props = defineProps<DatePickerAnchorProps>();
+
+const ctx = useDatePickerRootContext();
+
+onBeforeMount(() => {
+  ctx.hasCustomAnchor.value = true;
+});
+onUnmounted(() => {
+  ctx.hasCustomAnchor.value = false;
+});
+</script>
+
+<template>
+  <PopperAnchor v-bind="props">
+    <slot />
+  </PopperAnchor>
+</template>