Converts domain event structs into event maps and back again.

Configured with the list of domain event modules the store knows about, so callers can append plain structs and get them back from queries, while the store persists a stable string in the type column:

Sourced.EventStore.new(
  adapter: Sourced.EventStore.Postgres,
  config: [repo: MyApp.Repo],
  middleware: [
    {Sourced.Middleware.Domain, [OrderPlaced, OrderCancelled]}
  ]
)

Returned Sourced.StoredEvents carry the event module in type — the stored string is an implementation detail of the store, so what goes in comes back out.

Each entry is a struct module using Sourced.Middleware.Domain.Event. Its stored type is the module's Sourced.Middleware.Domain.Event.to_type/0, which defaults to the full module name (MyApp.OrderPlaced becomes "MyApp.OrderPlaced") and can be overridden on the module itself.

Appending a bare struct also derives its tags from Sourced.Middleware.Domain.Event.to_tags/1, so queries can match the event by the domain concepts it concerns rather than by type alone. A caller-built event map keeps whatever tags it was given.

Sourced.Middleware.init/1 validates the list when the store is built: every module must implement that behaviour and define a struct, and modules and types must be unique. It also builds the type lookup tables once, so no work is repeated per operation.

See Sourced.EventStore for the append and query ergonomics this enables.