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/views/Home.vue

@@ -0,0 +1,32 @@
+<script setup lang="ts">
+import { demos } from '../router';
+</script>
+
+<template>
+  <section class="max-w-3xl">
+    <h1 class="mb-2 mt-0 text-2xl font-semibold">
+      @robonen/primitives playground
+    </h1>
+    <p class="text-neutral-500 dark:text-neutral-400">
+      Pick a demo from the sidebar, or drop a new <code class="rounded border border-neutral-200 bg-white px-1.5 py-px text-xs dark:border-neutral-800 dark:bg-neutral-900">.vue</code> file into
+      <code class="rounded border border-neutral-200 bg-white px-1.5 py-px text-xs dark:border-neutral-800 dark:bg-neutral-900">src/demos/</code> — it will be picked up automatically and become
+      addressable at <code class="rounded border border-neutral-200 bg-white px-1.5 py-px text-xs dark:border-neutral-800 dark:bg-neutral-900">/demo/&lt;FileName&gt;</code>.
+    </p>
+
+    <div class="mt-4 grid gap-2 grid-cols-[repeat(auto-fill,minmax(11.25rem,1fr))]">
+      <RouterLink
+        v-for="demo in demos"
+        :key="demo.name"
+        :to="demo.routePath"
+        class="grid gap-1 rounded-lg border border-neutral-200 bg-white p-3 no-underline hover:border-blue-600 dark:border-neutral-800 dark:bg-neutral-900 dark:hover:border-blue-400"
+      >
+        <strong>{{ demo.name }}</strong>
+        <code class="text-neutral-500 dark:text-neutral-400">{{ demo.routePath }}</code>
+      </RouterLink>
+    </div>
+
+    <p v-if="!demos.length" class="text-neutral-500 dark:text-neutral-400">
+      No demos found yet.
+    </p>
+  </section>
+</template>