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/styles.css

@@ -0,0 +1,12 @@
+@import "tailwindcss";
+
+@layer base {
+  html,
+  body,
+  #app {
+    @apply m-0 h-full bg-neutral-50 text-sm text-neutral-900 antialiased dark:bg-neutral-950 dark:text-neutral-100;
+    color-scheme: light dark;
+  }
+}
+
+