Fix compilation failure when used as a hex dependency

[Copilot]

Elixir expands %Module{} struct literals at compile time via Module.__struct__/1. Because Mix compiles hex dependencies in parallel, cratis_chronicle_contracts modules are not yet loaded when chronicle is compiled, causing failures like:

error: Cratis.Chronicle.Contracts.Events.EventType.__struct__/1 is undefined, cannot expand struct Cratis.Chronicle.Contracts.Events.EventType.

Changes

• All external struct literals → struct/2 calls: Replaced every %ContractModule{...} literal from cratis_chronicle_contracts and bcl with struct(ContractModule, ...). struct/2 is resolved at runtime, eliminating the compile-time dependency on those modules being loaded.

  # Before
  %EventType{Id: id, Generation: gen}
  
  # After
  struct(EventType, Id: id, Generation: gen)

• @mongodb_sink_type_id module attribute → private function (projections/registrar.ex, reducers/handler.ex): Module attributes are evaluated at compile time, so the %BclGuid{...} literal was moved into a defp mongodb_sink_type_id/0 that returns the struct at runtime.

  # Before
  @mongodb_sink_type_id %BclGuid{lo: 0x45472BE122202C41, hi: 0x75FD97F7B1F0009C}
  
  # After
  defp mongodb_sink_type_id, do: struct(BclGuid, lo: 0x45472BE122202C41, hi: 0x75FD97F7B1F0009C)