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/navigation-menu/utils.ts

@@ -0,0 +1,51 @@
+/**
+ * Returns the open state string for the current item value vs the active menu value.
+ */
+export function getOpenState(value: string, itemValue: string): 'open' | 'closed' {
+  return value === itemValue ? 'open' : 'closed';
+}
+
+export function makeTriggerId(baseId: string, value: string): string {
+  return `${baseId}-trigger-${value}`;
+}
+
+export function makeContentId(baseId: string, value: string): string {
+  return `${baseId}-content-${value}`;
+}
+
+/** Only call `handler` when the pointer device is a mouse. */
+export function whenMouse<E extends PointerEvent>(handler: (event: E) => void): (event: E) => void {
+  return (event: E) => {
+    if (event.pointerType === 'mouse') handler(event);
+  };
+}
+
+/**
+ * Temporarily removes elements from the tab order while content is closed, returning
+ * a restore function. Used so background content keeps its tabindex when re-opened.
+ */
+export function removeFromTabOrder(candidates: HTMLElement[]): () => void {
+  for (const c of candidates) {
+    c.dataset['tabindex'] = c.getAttribute('tabindex') ?? '';
+    c.setAttribute('tabindex', '-1');
+  }
+  return () => {
+    for (const c of candidates) {
+      const prev = c.dataset['tabindex'] ?? '';
+      if (prev === '') c.removeAttribute('tabindex');
+      else c.setAttribute('tabindex', prev);
+    }
+  };
+}
+
+export function clamp(value: number, min: number, max: number): number {
+  return Math.min(Math.max(value, min), max);
+}
+
+/** Selector identifying the link/item nodes for arrow navigation inside content. */
+export const COLLECTION_ITEM_ATTR = 'data-primitives-collection-item';
+
+/** Custom event dispatched by a `NavigationMenuLink` selection. */
+export const LINK_SELECT_EVENT = 'navigationMenu.linkSelect';
+/** Custom event bubbled to the root content when an item dismisses the menu. */
+export const EVENT_ROOT_CONTENT_DISMISS = 'navigationMenu.rootContentDismiss';