Capture & store reference serve-receive poses

[HagenFritz]

Populate baseline “gold” vectors.

• Film/photograph at least 5 best serve-receive frames (back-view)
• Run extractor → normaliser → features on each; average angle vectors.
• Save as references/serve_receive_back.json with meta {view: "back", samples: 5}.
• Add capture notes to /docs/