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/StepperItem.vue

@@ -0,0 +1,63 @@
+<script lang="ts">
+import type { PrimitiveProps } from '../primitive';
+
+export interface StepperItemProps extends PrimitiveProps {
+  /** 1-based index associating this item with a step. */
+  step: number;
+  /** Disable this specific step. */
+  disabled?: boolean;
+  /** Mark the step as completed regardless of current `modelValue`. */
+  completed?: boolean;
+}
+</script>
+
+<script setup lang="ts">
+import { computed, toRef } from 'vue';
+import { provideStepperItemContext, useStepperRootContext } from './context';
+import { Primitive } from '../primitive';
+import { useForwardExpose } from '@robonen/vue';
+import { useId } from '../config-provider';
+
+const { as = 'div', step, disabled = false, completed = false } = defineProps<StepperItemProps>();
+
+const root = useStepperRootContext();
+const { forwardRef } = useForwardExpose();
+
+const state = computed(() => {
+  if (completed) return 'completed' as const;
+  if (root.value.value === step) return 'active' as const;
+  if (root.value.value > step) return 'completed' as const;
+  return 'inactive' as const;
+});
+
+const focusable = computed(() => {
+  if (disabled || root.disabled.value) return false;
+  if (!root.linear.value) return true;
+  return step <= root.value.value + 1;
+});
+
+const titleId = useId(undefined, 'stepper-item-title').value;
+const descriptionId = useId(undefined, 'stepper-item-description').value;
+
+provideStepperItemContext({
+  step: toRef(() => step),
+  state,
+  disabled: toRef(() => disabled || root.disabled.value),
+  focusable,
+  titleId,
+  descriptionId,
+});
+</script>
+
+<template>
+  <Primitive
+    :ref="forwardRef"
+    :as="as"
+    :aria-current="state === 'active' ? 'step' : undefined"
+    :data-state="state"
+    :data-orientation="root.orientation.value"
+    :data-disabled="disabled || root.disabled.value || !focusable ? '' : undefined"
+  >
+    <slot :state="state" :step="step" />
+  </Primitive>
+</template>