Copy: pare product claims to what we can certify, cut rule-of-three cadence, record arch 10 §10

src/app/components/Onboarding.tsx

@@ -137,8 +137,8 @@ export function Onboarding() {
       <div className={styles.inner}>
         <h2 className={styles.title}>Welcome to Astrolabe</h2>
         <p className={styles.tagline}>
-          A local library for your Vega-Lite charts — authored as JSON, rendered live, and kept on
-          your device.
+          A local library for your Vega-Lite charts — write the spec as JSON, watch it render live,
+          and keep it all on your device.
         </p>
 
         <div className={styles.ctaRow}>