Process Stepper Accordion

Build a "Process Stepper Accordion" React component.

CONCEPT: A 'How we work' section: four process steps on a vertical line. Selecting a step expands its details and fills the progress line from the top down to that step.

LAYOUT & CONTENT:
- Steps: Listen, Sketch, Build, Launch — each with a 2-sentence description and a duration chip (e.g. '1 week').
- Left rail: a 1px charcoal line with a cobalt fill overlay; each step has a 20px node circle that becomes a cobalt check when passed.

INTERACTIONS & MOTION:
- Rail fill scaleY animates to (activeIndex / (steps-1)) with origin-top spring stiffness 200 damping 28.
- Nodes before/at active animate background to cobalt and show a check (scale 0 → 1).
- Detail panel expands with height auto animation; others collapse.

DESIGN SYSTEM (non-negotiable):
- Palette: paper background #FAF9F6, deep charcoal #121212 for text and 1px borders, ONE warm accent: cobalt #1D4ED8 or terracotta #D9534F. Secondary surface #F2EFE8.
- Typography: Swiss / editorial. A tight grotesk for UI (Instrument Sans, Inter Tight or similar) with negative tracking on display sizes; an italic serif (Instrument Serif) used sparingly for one emphasized word.
- NO dark-mode neon, NO glow, NO glassmorphism, NO backdrop blur, NO purple-to-blue gradients, NO emoji icons. Solid fills, hairline rules, honest shadows (hard offset or none).
- Motion: Framer Motion with spring physics (type: "spring", stiffness 300-500, damping 20-35). Animate transform and opacity only. Respect prefers-reduced-motion via useReducedMotion().
- Icons: lucide-react, 1.5px stroke.
- Accessibility: real <button>/<a>/<label> elements, visible focus ring (2px cobalt, 3px offset), keyboard operable, aria attributes where state changes.

OUTPUT: one self-contained React component file (Tailwind CSS v4 utility classes, framer-motion, lucide-react), no external images, default export, ready to paste into a Next.js App Router page with "use client".