[WIP] Introduce context-first configuration and register Unitsdb model classes

[suleman-uzair]

Summary

This change introduces a context-first Unitsdb::Configuration flow and wires model registration through Configuration.register_model.

What changed

• Added Unitsdb::Configuration with:
  • register_model
  • context / populate_context
  • register / populate_register
  • substitution resolution for symbol/class substitution input
• Updated Unitsdb.database to be context-aware and cache by context ID.
• Updated Unitsdb::Database.from_db to:
  • accept context:
  • load within GlobalContext.with_context(context_id)
  • pass register: only when an explicit register exists
• Added Configuration.register_model(...) entries across Unitsdb model files, including:
  • core serializable models
  • identifier/reference/root-derived models
  • QUDT and UCUM model classes
• Added focused context support spec:
  • validates custom context substitution path with explicit register population

Current behavior/design intent

• Context is first-class.
• Register remains explicit (not auto-created).
• No self-healing/stale auto-refresh logic.