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/DatePickerTrigger.vue

@@ -0,0 +1,41 @@
+<script lang="ts">
+import type { PrimitiveProps } from '../primitive';
+
+export interface DatePickerTriggerProps extends PrimitiveProps {}
+</script>
+
+<script setup lang="ts">
+import { useForwardExpose } from '@robonen/vue';
+import { onMounted } from 'vue';
+import { PopperAnchor } from '../popper';
+import { Primitive } from '../primitive';
+import { useDatePickerRootContext } from './context';
+
+const { as = 'button' } = defineProps<DatePickerTriggerProps>();
+
+const ctx = useDatePickerRootContext();
+const { forwardRef, currentElement } = useForwardExpose();
+
+onMounted(() => {
+  ctx.triggerElement.value = currentElement.value;
+});
+</script>
+
+<template>
+  <component :is="ctx.hasCustomAnchor.value ? Primitive : PopperAnchor" as="template">
+    <Primitive
+      :id="ctx.triggerId.value"
+      :ref="forwardRef"
+      :as="as"
+      :type="as === 'button' ? 'button' : undefined"
+      aria-haspopup="dialog"
+      :aria-expanded="ctx.open.value"
+      :aria-controls="ctx.contentId.value"
+      :data-state="ctx.open.value ? 'open' : 'closed'"
+      :data-primitives-date-picker-trigger="''"
+      @click="ctx.onOpenToggle"
+    >
+      <slot />
+    </Primitive>
+  </component>
+</template>