feat(website): add payoff captions to joins/relationship-diagram/sql-echo

Use the ADR-0047 Ctrl+]-delimited demo caption to narrate the payoff
moment of three casts with a neutral one-liner (no key names): the join
result, the relationship diagram, and the m:n junction expansion. Add a
`caption` step kind to the cast generator. Captions show at the climax
during playback and clear as the cast quits.

website/casts-src/generate.mjs

@@ -74,6 +74,19 @@ function keysFor(steps) {
       if (step.after != null) keys.push(`${step.after}ms`);
       continue;
     }
+    if (step.caption != null) {
+      // Demo step caption (ADR-0047 D3): `Ctrl+]` opens a stealth capture
+      // buffer, the characters accumulate invisibly (they don't touch the
+      // input line), and a second `Ctrl+]` commits them to the floating
+      // caption box. The caption persists until the next keystroke, so the
+      // trailing wait holds it on screen before the cast ends. Keep caption
+      // text plain ASCII — it is typed character-by-character like any input.
+      keys.push(JSON.stringify('^]')); // open capture
+      for (const ch of step.caption) keys.push(charKey(ch));
+      keys.push(JSON.stringify('^]')); // commit the caption
+      if (step.after != null) keys.push(`${step.after}ms`);
+      continue;
+    }
     throw new Error(`unrecognised step: ${JSON.stringify(step)}`);
   }
   return keys;