feat(website): refine casts — trim shell, autoplay+loop landing, cap size

Address cast review feedback:

- Trim every cast to the in-app region (generate.mjs): the recording now
  starts with the app already running and ends on the last in-app frame —
  drops the `$ rdbms-playground` launch and the return-to-shell frame (the
  latter was the stray cursor-under-$ artifact). Opt out per cast with
  `keepShell: true` for demos that document the CLI launch.
- Landing quickstart cast: autoPlay + loop, with a 2.5s hold on the final
  frame so it pauses before restarting.
- Cap the demo at max-width 46rem and centre it, so the player (fit:'width')
  no longer scales its font up to the full splash column.

Casts re-recorded via `pnpm casts`. Build clean (25 pages).

Tab-keypress visibility deferred to an in-app overlay primitive (filed as
issue #22 — also serves the planned guided-lesson system); the cast notes
Tab in its caption for now.

website/casts-src/casts.mjs

@@ -45,6 +45,7 @@ export const casts = [
     width: 90,
     height: 26,
     typeSpeed: '45ms',
+    holdEnd: 2.5, // landing cast loops — pause on the final frame before restart
     steps: [
       { wait: 1100 },
       { type: 'create table authors with pk', after: 1000 },

website/casts-src/generate.mjs

@@ -15,7 +15,7 @@
  * recording shows a clean `$ rdbms-playground` launch line.
  */
 import { spawnSync } from 'node:child_process';
-import { mkdirSync, writeFileSync, rmSync } from 'node:fs';
+import { mkdirSync, writeFileSync, readFileSync, rmSync } from 'node:fs';
 import { dirname, resolve } from 'node:path';
 import { fileURLToPath } from 'node:url';
 import os from 'node:os';
@@ -67,6 +67,45 @@ function keysFor(steps) {
   return keys;
 }
 
+/**
+ * Trim a recorded cast to just the in-app portion and (optionally) hold the
+ * final frame. The app enters the alternate screen (`?1049h`) right after
+ * launch and leaves it (`?1049l`) on quit, so keeping the events between those
+ * drops the shell prompt, the `$ rdbms-playground` launch line, and the
+ * return-to-shell frame — the cast starts with the app already running and
+ * ends on the last in-app frame (no stray cursor-under-prompt frame).
+ * `holdEnd` re-emits the final frame after a pause so a looping cast lingers
+ * before restarting.
+ */
+function trimCast(outPath, { holdEnd = 1.5 } = {}) {
+  const lines = readFileSync(outPath, 'utf8').split('\n').filter(Boolean);
+  const header = JSON.parse(lines[0]);
+  const events = lines.slice(1).map((l) => JSON.parse(l));
+  const isOut = (e) => Array.isArray(e) && e[1] === 'o' && typeof e[2] === 'string';
+
+  const start = events.findIndex((e) => isOut(e) && e[2].includes('?1049h'));
+  let end = -1;
+  for (let i = events.length - 1; i >= 0; i--) {
+    if (isOut(events[i]) && events[i][2].includes('?1049l')) {
+      end = i;
+      break;
+    }
+  }
+  const kept = events.slice(start < 0 ? 0 : start, end < 0 ? events.length : end);
+  if (kept.length === 0) return; // nothing matched — leave the cast untouched
+  const t0 = kept[0][0];
+  const rebased = kept.map((e) => [Number((e[0] - t0).toFixed(6)), e[1], e[2]]);
+  if (holdEnd > 0) {
+    const lastT = rebased[rebased.length - 1][0];
+    rebased.push([Number((lastT + holdEnd).toFixed(6)), 'o', '']); // hold final frame
+  }
+  delete header.duration; // let the player recompute from events
+  writeFileSync(
+    outPath,
+    [JSON.stringify(header), ...rebased.map((e) => JSON.stringify(e))].join('\n') + '\n'
+  );
+}
+
 function yamlFor(cast) {
   const keys = keysFor(cast.steps)
     .map((k) => `      - ${k}`)
@@ -113,6 +152,10 @@ for (const cast of selected) {
     console.error(`✗ ${cast.name} failed (autocast exit ${res.status})`);
     failures += 1;
   } else {
+    // Trim the shell launch/quit so the cast starts with the app already
+    // running (the default; opt out with `keepShell: true` for casts that
+    // deliberately document the CLI launch).
+    if (!cast.keepShell) trimCast(outPath, { holdEnd: cast.holdEnd ?? 1.5 });
     console.log(`✓ ${cast.name}`);
   }
 }