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/DatePickerField.vue

@@ -0,0 +1,67 @@
+<script lang="ts">
+import type { PrimitiveProps } from '../primitive';
+
+export interface DatePickerFieldProps extends PrimitiveProps {
+  /** Allow typing into the field. @default false (read-only display) */
+  editable?: boolean;
+  /** Display format for the rendered value. */
+  format?: Intl.DateTimeFormatOptions;
+  /** Placeholder text shown when no value is selected. */
+  placeholderText?: string;
+}
+</script>
+
+<script setup lang="ts">
+import { computed, ref, watch } from 'vue';
+import { formatDate } from '../calendar';
+import { useDatePickerRootContext } from './context';
+
+const {
+  as: _as = 'input',
+  editable = false,
+  format = { year: 'numeric', month: '2-digit', day: '2-digit' },
+  placeholderText,
+} = defineProps<DatePickerFieldProps>();
+
+const ctx = useDatePickerRootContext();
+
+const displayValue = computed(() => {
+  if (!ctx.modelValue.value) return '';
+  return formatDate(ctx.modelValue.value, format, ctx.locale.value);
+});
+
+const draft = ref(displayValue.value);
+watch(displayValue, (v) => {
+  draft.value = v;
+});
+
+function commit() {
+  if (!editable) return;
+  const text = draft.value.trim();
+  if (!text) {
+    ctx.modelValue.value = undefined;
+    return;
+  }
+  const parsed = new Date(text);
+  if (!Number.isNaN(parsed.getTime()))
+    ctx.modelValue.value = new Date(parsed.getFullYear(), parsed.getMonth(), parsed.getDate());
+  else
+    draft.value = displayValue.value;
+}
+
+function handleKeydown(e: KeyboardEvent) {
+  if (e.key === 'Enter') commit();
+}
+</script>
+
+<template>
+  <input
+    :value="editable ? draft : displayValue"
+    :readonly="!editable"
+    :placeholder="placeholderText"
+    :data-primitives-date-picker-field="''"
+    @input="(e) => { if (editable) draft = (e.target as HTMLInputElement).value; }"
+    @blur="commit"
+    @keydown="handleKeydown"
+  >
+</template>