feat: add LibGen direct download provider

[DeusExTaco]

Summary

• add the provider-neutral fingerprint and editable source-origin contract from LG-1
• add the separately packaged, stateless LibGen provider with bounded HTML discovery and keyed metadata enrichment
• add request-scoped resolver handling, same-source generic HTTPS resolution, bounded failover, and redacted health telemetry
• add a hardened non-root image plus scan, smoke, multi-architecture, and independent release workflow support

Validation

• make validate (407 passed, 1 skipped, 90.21% coverage)
• make security-check
• make docker-conformance
• make docker-source-smoke

Release scope

This PR does not publish or register a LibGen image. Pullbox integration, public-domain transfer validation, soak testing, and the explicit release decision remain in LG-3.

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[chatgpt-codex-connector]

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