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/CalendarNext.vue

@@ -0,0 +1,45 @@
+<script lang="ts">
+import type { PrimitiveProps } from '../primitive';
+
+export interface CalendarNextProps extends PrimitiveProps {
+  /** Override the root's `nextPage` for just this button. */
+  nextPage?: (placeholder: Date) => Date;
+}
+</script>
+
+<script setup lang="ts">
+import { computed } from 'vue';
+import { Primitive } from '../primitive';
+import { useCalendarRootContext } from './context';
+
+const { as = 'button', nextPage: nextPageProp } = defineProps<CalendarNextProps>();
+
+defineSlots<{
+  default?: (props: { disabled: boolean }) => unknown;
+}>();
+
+const ctx = useCalendarRootContext();
+const disabled = computed(() => ctx.disabled.value || ctx.isNextButtonDisabled(nextPageProp));
+
+function handleClick() {
+  if (disabled.value) return;
+  ctx.nextPage(nextPageProp);
+}
+</script>
+
+<template>
+  <Primitive
+    :as="as"
+    :type="as === 'button' ? 'button' : undefined"
+    aria-label="Next"
+    :aria-disabled="disabled || undefined"
+    :data-primitives-calendar-next="''"
+    :data-disabled="disabled ? '' : undefined"
+    :disabled="as === 'button' ? disabled : undefined"
+    @click="handleClick"
+  >
+    <slot :disabled="disabled">
+      Next
+    </slot>
+  </Primitive>
+</template>