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/editable/EditablePreview.vue

@@ -0,0 +1,53 @@
+<script lang="ts">
+import type { PrimitiveProps } from '../primitive';
+
+export interface EditablePreviewProps extends PrimitiveProps {}
+</script>
+
+<script setup lang="ts">
+import { Primitive } from '../primitive';
+import { computed } from 'vue';
+import { useEditableContext } from './context';
+import { useForwardExpose } from '@robonen/vue';
+
+const { as = 'span' } = defineProps<EditablePreviewProps>();
+
+const ctx = useEditableContext();
+const { forwardRef } = useForwardExpose();
+
+const text = computed(() => ctx.modelValue.value || ctx.placeholder.value.preview);
+const showPlaceholder = computed(() => ctx.isEmpty.value);
+
+function onFocus(): void {
+  if (ctx.activationMode.value === 'focus') ctx.edit();
+}
+
+function onDoubleClick(): void {
+  if (ctx.activationMode.value === 'dblclick') ctx.edit();
+}
+</script>
+
+<template>
+  <Primitive
+    :ref="forwardRef"
+    :as="as"
+    tabindex="0"
+    :hidden="ctx.autoResize.value ? undefined : ctx.isEditing.value"
+    :data-placeholder-shown="showPlaceholder ? '' : undefined"
+    :data-state="ctx.isEditing.value ? 'edit' : 'preview'"
+    :data-disabled="ctx.disabled.value ? '' : undefined"
+    :data-readonly="ctx.readonly.value ? '' : undefined"
+    :style="ctx.autoResize.value ? {
+      whiteSpace: 'pre',
+      userSelect: 'none',
+      gridArea: '1 / 1 / auto / auto',
+      visibility: ctx.isEditing.value ? 'hidden' : undefined,
+      overflow: 'hidden',
+      textOverflow: 'ellipsis',
+    } : undefined"
+    @focusin="onFocus"
+    @dblclick="onDoubleClick"
+  >
+    <slot>{{ text }}</slot>
+  </Primitive>
+</template>