Add Stienhardt Diamond MCP

[JacobiusMakes]

Adds a fully validated registry record for Stienhardt Diamond MCP.

• Ten documented read-only tools
• Public MIT repository
• Local npm and hosted Streamable HTTP transports
• No authentication required
• Sourced diamond education, grading-report guidance, gemology reference, face-up size estimates, and live inventory
• Campaign-tagged product links for measurable commerce attribution

Validation: python validate.py passed all 218 registry files.