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/casts.mjs

@@ -10,6 +10,8 @@
  *   { type: 'text', after: ms }       — type the text + press Enter, then pause
  *   { type: 'text', enter: false }    — type without pressing Enter
  *   { key: 'Tab'|'Enter'|'CtrlC'|'CtrlO'|'Esc', after } — press a single named key
+ *   { caption: 'text', after: ms }    — pop a demo caption (ADR-0047) bottom-right,
+ *                                       then hold it (it clears on the next key)
  *
  * Every cast must end by quitting the app so the driver returns to the shell
  * prompt — use `{ key: 'CtrlC' }` (Ctrl-C → quit), NOT a typed `quit`: Ctrl-C
@@ -72,6 +74,7 @@ export const casts = [
       // split type + Enter so the command reads before the long echo lands.
       { type: 'create m:n relationship from books to tags', enter: false, after: 1700 },
       { key: 'Enter', after: 3200 },
+      { caption: 'One command builds an entire junction table.', after: 3200 },
       { key: 'CtrlC' }, // quit invisibly (Ctrl-C) — cast ends on the last content frame
     ],
   },
@@ -180,6 +183,7 @@ export const casts = [
         type: 'select authors.name, books.title from books join authors on books.author_id = authors.author_id order by authors.name',
         after: 2600,
       },
+      { caption: 'Each book paired with its author, joined across the two tables.', after: 3200 },
       { key: 'CtrlC' }, // quit invisibly (Ctrl-C) — cast ends on the last content frame
     ],
   },
@@ -202,6 +206,7 @@ export const casts = [
       },
       // the money shot: the two-table connector diagram
       { type: 'show relationship books_author', after: 2600 },
+      { caption: 'Many books, each pointing at one author.', after: 3200 },
       { key: 'CtrlC' }, // quit invisibly (Ctrl-C) — cast ends on the last content frame
     ],
   },