feat(website): projects cast (vi-nav load picker) + --demo on all casts

- New projects cast: create → save as library → new (fresh) → load → navigate
  the picker to the saved project (j, now possible via #24 vi-nav) → Enter
  loads it, the table is restored. Runs under an isolated --data-dir so the
  picker lists only this cast's projects.
- Turn on the demonstration overlay (--demo, #22 / ADR-0047) for ALL casts,
  for a consistent viewer experience: special keys show a badge — e.g.
  [ENTER], and [TAB] at the assistive-editor's completion moment, finally
  making that keystroke visible. Plain j/k navigation stays unbadged, so the
  picker navigation is not surfaced.
- Generator: per-cast `dataDir` (isolated data root) + default-on `--demo`
  (opt out with demo:false). All 7 casts regenerated.

Convert projects.md → .mdx and embed. Build clean (26 pages). Visual playback
of all casts pending a tunnel check.

website/casts-src/generate.mjs

@@ -110,10 +110,18 @@ function trimCast(outPath, { holdEnd = 1.5 } = {}) {
   );
 }
 
-function yamlFor(cast) {
+function yamlFor(cast, dataDir) {
   const keys = keysFor(cast.steps)
     .map((k) => `      - ${k}`)
     .join('\n');
+  // Build the launch command. `--demo` turns on the demonstration overlay
+  // (ADR-0047): automatic keystroke badges (so otherwise-invisible keys like
+  // `j` / Tab / Enter are shown) plus `Ctrl+]`-delimited step captions.
+  // `--data-dir` isolates the recording from the user's real projects (and
+  // keeps the load picker listing only this cast's projects).
+  const cmd = ['rdbms-playground'];
+  if (cast.demo !== false) cmd.push('--demo'); // on for all casts (opt out with demo:false)
+  if (dataDir) cmd.push('--data-dir', dataDir);
   return [
     'settings:',
     `  width: ${cast.width ?? 90}`,
@@ -124,7 +132,7 @@ function yamlFor(cast) {
     '  prompt: "$ "',
     'instructions:',
     '  - !Interactive',
-    '    command: rdbms-playground',
+    `    command: ${cmd.join(' ')}`,
     '    keys:',
     keys,
     '',
@@ -145,7 +153,16 @@ let failures = 0;
 for (const cast of selected) {
   const yamlPath = resolve(os.tmpdir(), `autocast-${cast.name}.yaml`);
   const outPath = resolve(outDir, `${cast.name}.cast`);
-  writeFileSync(yamlPath, yamlFor(cast));
+  // Per-cast isolated data root (so the load picker lists only this cast's
+  // projects and nothing touches the user's real data dir). Wiped fresh each
+  // run for a deterministic picker order.
+  let dataDir = null;
+  if (cast.dataDir) {
+    dataDir = resolve(os.tmpdir(), `rdbms-cast-data-${cast.name}`);
+    rmSync(dataDir, { recursive: true, force: true });
+    mkdirSync(dataDir, { recursive: true });
+  }
+  writeFileSync(yamlPath, yamlFor(cast, dataDir));
   console.log(`▶ recording ${cast.name} → public/casts/${cast.name}.cast`);
   const res = spawnSync('autocast', ['--overwrite', yamlPath, outPath], {
     env,