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/env.d.ts

@@ -1 +1,17 @@
-declare const __DEV__: boolean;
+export {};
+
+declare global {
+  const __DEV__: boolean;
+}
+
+declare module 'vue' {
+  interface ComponentCustomProps {
+    [key: `data${string}`]: unknown;
+  }
+}
+
+declare module 'vue' {
+  interface HTMLAttributes {
+    [key: `data-${string}`]: unknown;
+  }
+}