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.src.json

@@ -0,0 +1,12 @@
+{
+  "extends": "@robonen/tsconfig/tsconfig.vue.json",
+  "compilerOptions": {
+    "composite": true,
+    "types": [],
+    "paths": {
+      "@/*": ["./src/*"]
+    },
+    "tsBuildInfoFile": "./node_modules/.tmp/tsconfig.src.tsbuildinfo"
+  },
+  "include": ["src/**/*.ts", "src/**/*.vue"]
+}