Replace conformance manifests with commit locks

[dcajal]

Summary

• replace downstream conformance manifests with one-line commit locks
• remove the implementation-manifest schema and external manifest validation from Biosiglib
• keep release coordination based on exact downstream commit pins

Why

Conformance is an invariant of merged implementation code, not a status carried by metadata. The only downstream datum Biosiglib needs for reproducibility is the exact tested commit.

Validation

• 14 validator unit tests
• full Biosiglib specification validation
• generated documentation check
• MkDocs strict build
• workflow YAML parsing
• git diff --check