docs: add AI-agent-readable schemaVersion 3/4 specification

[webwarrior06]

Adds SCHEMA_VERSION_SPEC.md — a structured specification document written for AI coding agents.

What this covers

• Glossary distinguishing schemaVersion, the entry-list version header, and formatVersion
• Full data model for RootHash, UpdatedRootHash, and the entry-list text format
• Table of every HTTP endpoint and whether it reads/sends schemaVersion
• Step-by-step runtime branching logic for getRootHash() and getEntries()
• Collection hash algorithm (distinct from file-content SHA-256)
• Explicit table of which high-level methods are safe under schemaVersion 4
• Statement that no migration logic exists
• Test coverage gaps (schemaVersion 4 error paths not tested)
• Actionable rules for agents making future changes (choke points, what must change together, what is safe to call independently)