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/README.md

@@ -0,0 +1,32 @@
+# @robonen/primitives playground
+
+Minimal Vite + Vue 3 sandbox for inspecting and debugging primitives from
+[`@robonen/primitives`](../). Imports source directly via the `@primitives/*`
+alias, so HMR works while editing components in `vue/primitives/src/`.
+
+## Usage
+
+```sh
+pnpm --filter @robonen/primitives-playground dev
+```
+
+Then open http://localhost:5180.
+
+## Adding a demo
+
+Drop a `.vue` file into `src/demos/`. It will be picked up automatically by the
+sidebar (`import.meta.glob('./demos/*.vue')`) and addressable via the URL hash
+(e.g. `#/Accordion`).
+
+```vue
+<script setup lang="ts">
+import { AccordionRoot } from '@primitives/accordion';
+// or: import { AccordionRoot } from '@primitives';
+</script>
+```
+
+## Notes
+
+- Imports resolve to `vue/primitives/src/` directly (not the built `dist/`), so
+  you can poke at primitives and see changes instantly.
+- The playground is `private: true` and is not published.