Add StepIndicator molecule — horizontal segmented progress bar

- Maps to Figma Progress Bar - Steps (2375:47468)
- Segmented bars: brand gold for completed/current, grey for incomplete
- Current step label bolded, responsive bar height (10px/6px)
- role="navigation" + aria-current="step" for accessibility
- 7 stories: Default, AllStates, TwoSteps, ManySteps, Interactive,
  Completed, NarrowContainer

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/molecules/StepIndicator/index.ts

@@ -0,0 +1,2 @@
+export { StepIndicator } from './StepIndicator';
+export type { StepIndicatorProps, Step } from './StepIndicator';