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/eslint.config.ts

@@ -0,0 +1,9 @@
+import { base, compose, imports, stylistic, typescript, vue } from '@robonen/eslint';
+
+export default compose(base, typescript, vue, imports, stylistic, {
+  name: 'primitives/overrides',
+  files: ['**/*.vue'],
+  rules: {
+    '@stylistic/no-multiple-empty-lines': 'off',
+  },
+});