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/toggle-group/context.ts

@@ -0,0 +1,25 @@
+import type { ComputedRef, Ref } from 'vue';
+import type { RovingDirection, RovingOrientation } from '../utils/roving-focus';
+import { useContextFactory } from '@robonen/vue';
+
+export type ToggleGroupType = 'single' | 'multiple';
+
+export interface ToggleGroupContext {
+  type: Ref<ToggleGroupType>;
+  value: Ref<string[]>;
+  toggle: (v: string) => void;
+  isPressed: (v: string) => boolean;
+  orientation: Ref<RovingOrientation>;
+  direction: Ref<RovingDirection>;
+  loop: Ref<boolean>;
+  disabled: Ref<boolean>;
+  rovingFocus: Ref<boolean>;
+  /** DOM-ordered items, sourced from the internal Collection. */
+  items: ComputedRef<HTMLElement[]>;
+  onItemKeyDown: (event: KeyboardEvent, el: HTMLElement) => void;
+}
+
+const ctx = useContextFactory<ToggleGroupContext>('ToggleGroupContext');
+
+export const provideToggleGroupContext = ctx.provide;
+export const useToggleGroupContext = ctx.inject;