fix(showcase): commit jumping-jacks clip so the Mixamo hero works in production

The Mixamo hero/playground clip never actually played in production: the
.fbx files are gitignored and there is no /clips CDN rewrite in
vercel.json, so /clips/jumping-jacks.fbx 404s and silently falls back to
procedural. (Verified: prod /clips/jumping-jacks.fbx -> 404, while the
committed /models/character.glb -> 200.)

Fix mirrors how character.glb is already handled — commit the single clip
the marketing surfaces use so Vercel serves it directly:

- .gitignore: negate the ignore for jumping-jacks.fbx only; the other five
  large, unused Mixamo clips stay ignored
- playground/public/clips/jumping-jacks.fbx: committed (375 KB, smaller
  than the already-committed 702 KB character.glb)
- playground/src/clips.ts: correct the stale 'served from CDN' comment

No CDN/infra required. typecheck, build, and tests all pass; the clip is
emitted into playground/dist/clips/ by the build.

Author: a-baran-orhan

.gitignore

@@ -11,3 +11,8 @@ coverage/
 playground/public/clips/*.fbx
 playground/public/clips/*.glb
 playground/public/clips/.DS_Store
+# Exception: the jumping-jacks showcase clip is committed so Vercel serves it
+# directly (like public/models/character.glb). Prod has no /clips CDN, so an
+# ignored clip 404s and silently falls back to procedural. Keep the other large
+# Mixamo clips ignored — they're unused on the marketing surfaces.
+!playground/public/clips/jumping-jacks.fbx

playground/public/clips/jumping-jacks.fbx

@@ -5,9 +5,11 @@
  * marketing surfaces, while the procedural DSL stays the source of truth for
  * every un-clipped movement.
  *
- * The FBX binaries live in `public/clips/` (gitignored, served from storage/CDN
- * in production). A movement plays its clip only when it declares `clip "<name>"`
- * AND the skinned character is active; anything missing falls back to procedural.
+ * The FBX binaries live in `public/clips/`. The `jumping-jacks` showcase clip is
+ * committed so Vercel serves it directly (production has no /clips CDN rewrite);
+ * the other, unused clips stay gitignored. A movement plays its clip only when it
+ * declares `clip "<name>"` AND the skinned character is active; anything missing
+ * (e.g. an un-committed clip that 404s in prod) falls back to procedural.
  */
 export const SHOWCASE_CLIPS: Record<string, string> = {
   walk: "/clips/walk.fbx",