docs: use resilient DOI badge image

[ViktorMikhalkin]

Summary

• replace the Zenodo-hosted SVG badge image with a Shields.io badge
• keep the badge linked to the Zenodo concept DOI

Root cause

Zenodo's dynamic SVG badge endpoints currently return HTTP 403, while the DOI records themselves remain valid. The Shields SVG renders successfully.

Scope

One documentation line only. No scientific metadata, manuscript, source code, release, tag, or DOI semantics changed.

Verification

• RED: current README failed the expected resilient-badge assertion
• GREEN: Shields URL present and old failing Zenodo SVG URL absent
• Shields SVG rendered successfully
• git diff --check
• independent review: no findings