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/dialog/DialogOverlay.vue

@@ -0,0 +1,36 @@
+<script lang="ts">
+import type { PrimitiveProps } from '../primitive';
+
+export interface DialogOverlayProps extends PrimitiveProps {
+  /**
+   * Keep overlay mounted even when the dialog is closed — useful for CSS
+   * exit animations.
+   * @default false
+   */
+  forceMount?: boolean;
+}
+</script>
+
+<script setup lang="ts">
+import { Presence } from '../presence';
+import { Primitive } from '../primitive';
+import { useDialogContext } from './context';
+import { useForwardExpose } from '@robonen/vue';
+
+const { as = 'div', forceMount = false } = defineProps<DialogOverlayProps>();
+const { forwardRef } = useForwardExpose();
+const ctx = useDialogContext();
+</script>
+
+<template>
+  <Presence v-if="ctx.modal.value" :present="ctx.open.value" :force-mount="forceMount">
+    <Primitive
+      :ref="forwardRef"
+      :as="as"
+      :data-state="ctx.open.value ? 'open' : 'closed'"
+      :style="{ pointerEvents: 'auto' }"
+    >
+      <slot />
+    </Primitive>
+  </Presence>
+</template>