Add The Unplayed Stage showcase demo

[hoainho]

Summary

• Demo id / title: unplayed-stage / The Unplayed Stage
• One-line summary: A fully procedural concert stage with piano, guitar, bass, drums, score-driven motion, native Motion controls, and four declared export scopes.
• Author (display name) / author URL: Hoài Nhớ / https://github.com/hoainho

Contributor checklist

• Reference image rights: original generated stage concept reference authored for this reconstruction
• Generated via the img2threejs skill (version: 1.5.1)
• npm run build passes locally
• Screenshot of the rendered demo attached below
• Only touched src/demos/unplayed-stage/, public/references/unplayed-stage.jpg, and appended to src/demos/registry.ts
• New registry.ts entry has every DemoEntry field filled and status: 'final'

Screenshot

Runtime QA also exercised both Electric Gravity · 132 s and Canon · 104 s, the stop-to-idle path, the 576-part inspector, and the four-model export scope. The local safety scan passed with no violations.

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Maintainer review (do not fill as contributor)

• Visual fidelity acceptable against the reference image
• License/rights statement plausible; no PII or trademark-as-primary-subject issue
• status: 'final' (not left as 'placeholder')
• Bundle size delta looks sane
• pr-safety-check is green