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/listbox/index.ts

@@ -0,0 +1,32 @@
+export { default as ListboxRoot } from './ListboxRoot.vue';
+export { default as ListboxContent } from './ListboxContent.vue';
+export { default as ListboxItem } from './ListboxItem.vue';
+export { default as ListboxItemIndicator } from './ListboxItemIndicator.vue';
+export { default as ListboxGroup } from './ListboxGroup.vue';
+export { default as ListboxGroupLabel } from './ListboxGroupLabel.vue';
+export { default as ListboxFilter } from './ListboxFilter.vue';
+
+export {
+  provideListboxRootContext,
+  useListboxRootContext,
+  provideListboxItemContext,
+  useListboxItemContext,
+  provideListboxGroupContext,
+  useListboxGroupContext,
+  type ListboxRootContext,
+  type ListboxItemContext,
+  type ListboxGroupContext,
+  type ListboxOrientation,
+  type ListboxDirection,
+  type ListboxSelectionBehavior,
+} from './context';
+
+export { compare as compareListboxValues, includes as includesListboxValue, type ListboxValue } from './utils';
+
+export type { ListboxRootProps, ListboxRootEmits } from './ListboxRoot.vue';
+export type { ListboxContentProps } from './ListboxContent.vue';
+export type { ListboxItemProps, ListboxItemEmits } from './ListboxItem.vue';
+export type { ListboxItemIndicatorProps } from './ListboxItemIndicator.vue';
+export type { ListboxGroupProps } from './ListboxGroup.vue';
+export type { ListboxGroupLabelProps } from './ListboxGroupLabel.vue';
+export type { ListboxFilterProps, ListboxFilterEmits } from './ListboxFilter.vue';