Static defaults for array embedded resource attributes cause a cryptic compile error.

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

### AI Policy

- [x] I agree to follow this project's AI Policy, or I agree that AI was not used while creating this issue.

### Versions

erlang 28.1
elixir 1.19.0-otp-28

### Operating system

macOS 26.0.1

### Current Behavior

Given a resource with an array embedded resource and static defaults:

```
attribute :channels, {:array, MyApp.Channel} do
default [%{name: :web, suppressible: false}, %{name: :email, suppressible: true}]
end
```

The app fails to compile with the following compilation error:

```
== Compilation error in file lib/my_app/notification_type/assignment_posted.ex ==
** (ArgumentError) cannot escape #Reference<0.96162257.3960733698.139143>. The supported values are: lists, tuples, maps, atoms, numbers, bitstrings, PIDs and remote functions in the format &Mod.fun/arity
(spark 2.3.5) lib/my_app/notification_type/assignment_posted.ex:1: Spark.Dsl.__before_compile__/1
```

This is caused by the `create` action on the `MyApp.Channel` being called **at compile time** via a bulk create resulting in the following extra metadata:

```
__metadata__: %{
:selected => [:name, :suppressible],
{:bulk_create_index, #Reference<0.910774993.731906049.251376>} => 1
}
```

this metadata is on the two created records which are trying to be saved as the `default` value of the attribute entity on the resource.

This change was caused by efcf04748e327e6ffa5e8b29705e8c414775b4a5 AFAICT.

The user has been advised to use the 0-arity function version of `defaults` so that the resources are created when the `:create` action is called on the parent resource, rather than at compile time.

I am opening this bug to track that there is a compile error that is very difficult for a user to debug and not the fact that the behaviour has changed, because I think the old behaviour was probably wrong.

My suggested resolution is to remove the creation of default embedded records at compile time and emit an error.

### Reproduction

_No response_

### Expected Behavior

_No response_

Start by tracing how static defaults for array embedded resources are processed before compilation, using the Spark.Dsl.__before_compile__ error and the shown __metadata__ reference as clues. The issue suggests removing compile-time creation of embedded records and emitting a clear error instead; verify the resulting behavior with the supplied attribute example.

Written by the indexing model from the issue text.