Add a glossary section for core project terms

[Permea-lab-admin]

Goal

Help new contributors understand recurring Permea Core terminology.

Suggested scope

• Add a small glossary section to an appropriate docs page, or create a short glossary doc if that fits the existing structure.
• Include terms such as benchmark, artifact, provenance, evidence ladder, claim boundary, and run manifest.
• Use neutral definitions tied to existing docs.

Acceptance criteria

• At least 6 terms are defined.
• Definitions are short and consistent with existing repository documents.
• The glossary is linked from a discoverable place.

Contributor note

This is a beginner-friendly docs issue for careful readers.