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/tsconfig.json

@@ -1,17 +1,7 @@
 {
-  "extends": "@robonen/tsconfig/tsconfig.json",
-  "compilerOptions": {
-    "lib": ["ESNext", "DOM"],
-    "baseUrl": ".",
-    "paths": {
-      "@/*": ["src/*"]
-    }
-  },
-  "vueCompilerOptions": {
-    "strictTemplates": true,
-    "fallthroughAttributes": true,
-    "inferTemplateDollarAttrs": true,
-    "inferTemplateDollarEl": true,
-    "inferTemplateDollarRefs": true
-  }
-}
+  "files": [],
+  "references": [
+    { "path": "./tsconfig.src.json" },
+    { "path": "./tsconfig.node.json" }
+  ]
+}