feat(showcase): Mixamo jumping-jacks hook on hero, playground default, README GIF

Re-enable the jumping-jacks clip-back on the three primary marketing
surfaces as a pure eye-candy hook, while every other preset and the
sub-3 README GIFs stay procedural (the DSL remains source of truth).

- spec/examples/jumping-jacks.posecode: add clip "jumping-jacks" (this
  single directive drives the landing hero, playground default, and the
  captured GIF, all of which read this doc + SHOWCASE_CLIPS)
- playground/src/main.ts: DEFAULT_PRESET = jumping-jacks so the
  playground greets visitors with the mocap clip; library order unchanged
- docs/media/jumping-jacks.gif: re-render from the clip

The clip directive stays visible in the editor, so the mechanism is
transparent; deleting it drops back to procedural.

Note: public/clips/*.fbx is gitignored (CDN-served); the prod asset must
be named jumping-jacks.fbx to match SHOWCASE_CLIPS.

Author: a-baran-orhan

docs/media/jumping-jacks.gif

@@ -19,6 +19,12 @@ import {
 import type { PosecodeEditor } from "./editor.js";
 import { PRESETS } from "./presets.js";
 import { SHOWCASE_CLIPS } from "./clips.js";
+
+// The movement shown on first open (no shared link). Jumping jacks plays the
+// Mixamo showcase clip, so the playground greets visitors with polished mocap
+// while every other preset — and the editor once you engage — stays procedural.
+// Falls back to the first library entry if the id is ever removed.
+const DEFAULT_PRESET = PRESETS.find((p) => p.id === "jumping-jacks") ?? PRESETS[0]!;
 import { renderWarnings } from "./warnings.js";
 import llmPrompt from "../../spec/llm-authoring.md?raw";
 
@@ -506,9 +512,9 @@ if (sharedSource) {
   initialDoc = sharedSource;
   intro.hidden = true;
 } else {
-  initialDoc = PRESETS[0]!.source;
-  currentPresetId = PRESETS[0]!.id;
-  libCurrent.textContent = PRESETS[0]!.label;
+  initialDoc = DEFAULT_PRESET.source;
+  currentPresetId = DEFAULT_PRESET.id;
+  libCurrent.textContent = DEFAULT_PRESET.label;
 }
 
 // Boot the two heavyweights (CodeMirror editor + Three.js renderer) after the

playground/src/main.ts

@@ -1,5 +1,6 @@
 posecode exercise "Jumping jacks"
   rig humanoid
+  clip "jumping-jacks"
   pose start = standing
 
   step "Out" 0.5s settle: