[DavidBadura]

This moves the 2.0 line to event-sourcing 4.0 (the 1.x line stays for 3.x) and brings the DCB parts back, which were removed from 2.0.x while they only existed in 4.0.

The first commit is the upgrade itself. Child aggregates are gone in 4.0, so the rules only look at aggregate roots now and the child aggregate fixtures are removed. The identifiers moved to the Identifier namespace, which touches the fixtures and the docs samples.

The second commit adds the type inference for patchlevel/event-sourcing#937, which replaced the decision model builder with a Decision:

$state = $decision->read([
    'nextInvoiceNumber' => new NextInvoiceNumberProjection(),
    'invoiceCount' => new InvoiceCountProjection(),
]); // array{nextInvoiceNumber: int, invoiceCount: int}

[$next, $count] = $decision->read([...]); // array{int, int}

Without the extension the array form is only array<K, mixed>. A single projection is already typed natively through the projection generic, the extension covers it too and falls back to the return type of initialState(), so projections without @extends BasicProjection<...> get their type as well. ProjectionBuilder::build() is typed the same way. Both are always on, they don't report anything, so they have no configuration switch. The docs get a "Typed decisions" section.