Add wraparound tidepool keyboard navigation

src/tidepool.js

@@ -0,0 +1,12 @@
+export const zones = ['shore', 'rocks', 'pool'];
+
+export function nextZone(currentIndex, direction) {
+  const step = direction === 'previous' ? -1 : 1;
+  return (currentIndex + step + zones.length) % zones.length;
+}
+
+export function directionForKey(key) {
+  if (key === 'ArrowLeft' || key === 'ArrowUp') return 'previous';
+  if (key === 'ArrowRight' || key === 'ArrowDown') return 'next';
+  return null;
+}