Add single-collection inheritance for documents

[DavidBadura]

Document classes can now form a hierarchy that shares one collection. The root carries a #[DiscriminatorMap] attribute mapping a stable string to each concrete class; subclasses inherit the collection, database, id property and indexes. Every stored document gets a discriminator field (_type by default) so the hydrator can reconstruct the concrete class on load.

The repository for the root class is polymorphic: it accepts any subclass on write and returns concrete types on read. A repository for a concrete class is scoped to its own discriminator value, so its queries, counts and deletes never touch sibling types, and writes reject foreign types with WrongClass.

The metadata factory walks the parent chain to find the #[Document] class, validates the map and merges the field mappings of the whole hierarchy so you can filter by inherited and subclass-only properties from the root. Conflicting field names across subclasses raise DiscriminatorFieldConflict.