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/index.ts

@@ -0,0 +1,27 @@
+export { default as StepperRoot } from './StepperRoot.vue';
+export { default as StepperItem } from './StepperItem.vue';
+export { default as StepperTrigger } from './StepperTrigger.vue';
+export { default as StepperIndicator } from './StepperIndicator.vue';
+export { default as StepperTitle } from './StepperTitle.vue';
+export { default as StepperDescription } from './StepperDescription.vue';
+export { default as StepperSeparator } from './StepperSeparator.vue';
+
+export {
+  provideStepperRootContext,
+  provideStepperItemContext,
+  useStepperRootContext,
+  useStepperItemContext,
+  type StepperRootContext,
+  type StepperItemContext,
+  type StepperState,
+  type StepperOrientation,
+  type StepperDirection,
+} from './context';
+
+export type { StepperRootProps, StepperRootEmits } from './StepperRoot.vue';
+export type { StepperItemProps } from './StepperItem.vue';
+export type { StepperTriggerProps } from './StepperTrigger.vue';
+export type { StepperIndicatorProps } from './StepperIndicator.vue';
+export type { StepperTitleProps } from './StepperTitle.vue';
+export type { StepperDescriptionProps } from './StepperDescription.vue';
+export type { StepperSeparatorProps } from './StepperSeparator.vue';