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/stepper/context.ts

@@ -0,0 +1,43 @@
+import type { ComputedRef, Ref } from 'vue';
+import { useContextFactory } from '@robonen/vue';
+
+export type StepperOrientation = 'horizontal' | 'vertical';
+export type StepperDirection = 'ltr' | 'rtl';
+export type StepperState = 'completed' | 'active' | 'inactive';
+
+export interface StepperRootContext {
+  /** Currently active step (1-based). */
+  value: Ref<number>;
+  /** Total registered items, tracked through the Collection. */
+  total: ComputedRef<number>;
+  /** Orientation of the stepper — drives arrow-key axis. */
+  orientation: Ref<StepperOrientation>;
+  /** Writing direction. */
+  direction: Ref<StepperDirection>;
+  /** When `true`, steps must be completed in order. */
+  linear: Ref<boolean>;
+  /** Whether the whole stepper is disabled. */
+  disabled: Ref<boolean>;
+
+  goToStep: (step: number) => void;
+  onTriggerKeyDown: (event: KeyboardEvent, el: HTMLElement) => void;
+}
+
+export interface StepperItemContext {
+  step: Ref<number>;
+  state: Ref<StepperState>;
+  disabled: Ref<boolean>;
+  focusable: Ref<boolean>;
+  titleId: string;
+  descriptionId: string;
+}
+
+export const {
+  inject: useStepperRootContext,
+  provide: provideStepperRootContext,
+} = useContextFactory<StepperRootContext>('stepper');
+
+export const {
+  inject: useStepperItemContext,
+  provide: provideStepperItemContext,
+} = useContextFactory<StepperItemContext>('stepper-item');