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/vite.config.ts

@@ -0,0 +1,31 @@
+import { URL, fileURLToPath } from 'node:url';
+import tailwindcss from '@tailwindcss/vite';
+import vue from '@vitejs/plugin-vue';
+import { defineConfig } from 'vite';
+
+export default defineConfig(({ mode }) => ({
+  plugins: [vue(), tailwindcss()],
+  define: {
+    __DEV__: JSON.stringify(mode !== 'production'),
+  },
+  resolve: {
+    alias: [
+      // Order matters: subpath alias must come before the bare-specifier one.
+      {
+        find: /^@primitives\/(.*)$/,
+        replacement: fileURLToPath(new URL('../src/$1', import.meta.url)),
+      },
+      {
+        find: /^@primitives$/,
+        replacement: fileURLToPath(new URL('../src/index.ts', import.meta.url)),
+      },
+    ],
+  },
+  server: {
+    port: 5180,
+    fs: {
+      // Allow importing from primitives source one level up.
+      allow: [fileURLToPath(new URL('../', import.meta.url))],
+    },
+  },
+}));