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/calendar/CalendarCell.vue

@@ -0,0 +1,43 @@
+<script lang="ts">
+import type { PrimitiveProps } from '../primitive';
+
+export interface CalendarCellProps extends PrimitiveProps {
+  /** The date this cell represents. */
+  date: Date;
+}
+</script>
+
+<script setup lang="ts">
+import { computed } from 'vue';
+import { Primitive } from '../primitive';
+import { useCalendarGridContext, useCalendarRootContext } from './context';
+import { isSameDay, isSameMonth } from './utils';
+
+const { as = 'td', date } = defineProps<CalendarCellProps>();
+
+const ctx = useCalendarRootContext();
+const gridCtx = useCalendarGridContext();
+
+const isSelected = computed(() => ctx.isDateSelected(date));
+const isDisabled = computed(() => ctx.isDateDisabled(date));
+const isUnavailable = computed(() => ctx.isDateUnavailable(date));
+const isOutsideView = computed(() => !isSameMonth(date, gridCtx.month.value));
+const isToday = computed(() => isSameDay(date, new Date()));
+</script>
+
+<template>
+  <Primitive
+    :as="as"
+    role="gridcell"
+    :aria-selected="isSelected ? true : undefined"
+    :aria-disabled="(isDisabled || isUnavailable) ? true : undefined"
+    :data-primitives-calendar-cell="''"
+    :data-selected="isSelected ? '' : undefined"
+    :data-disabled="isDisabled ? '' : undefined"
+    :data-unavailable="isUnavailable ? '' : undefined"
+    :data-outside-view="isOutsideView ? '' : undefined"
+    :data-today="isToday ? '' : undefined"
+  >
+    <slot />
+  </Primitive>
+</template>