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/playground/src/demos/Checkbox.vue

@@ -0,0 +1,40 @@
+<script setup lang="ts">
+import { ref } from 'vue';
+import type { CheckedState } from '@primitives/checkbox';
+import { CheckboxIndicator, CheckboxRoot } from '@primitives/checkbox';
+
+const checked = ref<CheckedState>(false);
+const disabled = ref(false);
+</script>
+
+<template>
+  <section class="grid max-w-md gap-4">
+    <div class="flex flex-wrap items-center gap-3 rounded-lg border border-neutral-200 bg-white px-3 py-2.5 dark:border-neutral-800 dark:bg-neutral-900">
+      <label class="inline-flex items-center gap-1.5">
+        <input v-model="disabled" type="checkbox"> disabled
+      </label>
+      <button
+        type="button"
+        class="rounded border border-neutral-200 bg-neutral-50 px-2 py-1 text-xs hover:border-blue-600 dark:border-neutral-800 dark:bg-neutral-950 dark:hover:border-blue-400"
+        @click="checked = 'indeterminate'"
+      >
+        set indeterminate
+      </button>
+      <output class="ml-auto font-mono text-xs text-neutral-500 dark:text-neutral-400">checked = {{ JSON.stringify(checked) }}</output>
+    </div>
+
+    <label class="inline-flex cursor-pointer items-center gap-2.5">
+      <CheckboxRoot
+        v-model:checked="checked"
+        :disabled="disabled"
+        class="inline-grid h-5 w-5 place-items-center rounded border border-neutral-200 bg-white data-[disabled]:cursor-not-allowed data-[disabled]:opacity-50 data-[state=checked]:border-blue-600 data-[state=checked]:bg-blue-600 data-[state=checked]:text-white data-[state=indeterminate]:border-blue-600 data-[state=indeterminate]:bg-blue-600 data-[state=indeterminate]:text-white focus-visible:outline-2 focus-visible:outline-offset-2 focus-visible:outline-blue-600 dark:border-neutral-800 dark:bg-neutral-900 dark:data-[state=checked]:border-blue-500 dark:data-[state=checked]:bg-blue-500 dark:data-[state=indeterminate]:border-blue-500 dark:data-[state=indeterminate]:bg-blue-500 dark:focus-visible:outline-blue-400"
+      >
+        <CheckboxIndicator class="text-[0.8125rem] leading-none">
+          <span v-if="checked === 'indeterminate'">–</span>
+          <span v-else-if="checked">✓</span>
+        </CheckboxIndicator>
+      </CheckboxRoot>
+      Accept terms and conditions
+    </label>
+  </section>
+</template>