Schema registry and upcasting for reckon-db

Provides schema versioning and automatic event transformation: - Register schemas for event types with version numbers - Define upcast functions to transform old versions to new - Auto-upcast events when reading from streams

Event schema evolution strategies: - Weak schema: No validation, just track versions - Strong schema: Validate against JSON Schema or custom validator - Tolerant reader: Accept old versions, upcast on demand

Usage:

  %% Register schema
  reckon_db_schema:register(my_store, <<"OrderPlaced">>, #{
      version => 2,
      upcast_from => #{
          1 => fun(V1Data) -> V1Data#{new_field => default} end
      }
  }).
 
  %% Read with upcasting
  {ok, Events} = reckon_db_streams:read(my_store, stream, 0, 100, forward),
  UpcastedEvents = reckon_db_schema:upcast(my_store, Events).