[Bug]: Implicit class from JEP 512 compact source files not registered in model (allTypes, rootPackage, declaredTypes all empty)

### Describe the bug

When parsing a JEP 512 compact source file (no class declaration, methods at top level) at
compliance level 25, Spoon creates the implicit CtClass wrapper internally but never registers
it in any of the model's lookup structures.

model.getAllTypes(), rootPackage.getPackage("test").getTypes(), and
compilationUnit.getDeclaredTypes() all return empty for the compact source file. The implicit
class is effectively invisible to any consumer iterating the model.

### Source code you are trying to analyze/transform

```Java
package test;

void main() {
System.out.println("hello from compact source");
}

void helper(int x) {
System.out.println(x);
}

int add(int a, int b) {
return a + b;
}
```

### Source code for your Spoon processing

```Java
import spoon.Launcher;
import spoon.reflect.CtModel;
import spoon.reflect.declaration.CtCompilationUnit;

Launcher launcher = new Launcher();
launcher.addInputResource("src/test/resources/CompactSource.java");
launcher.getEnvironment().setComplianceLevel(25);
launcher.getEnvironment().setNoClasspath(true);
launcher.buildModel();

CtModel model = launcher.getModel();

// Expected: [CompactSource(implicit=true)]
// Actual: []
System.out.println("getAllTypes(): " + model.getAllTypes().stream()
.map(t -> t.getSimpleName() + "(implicit=" + t.isImplicit() + ")")
.toList());

// Expected: [CompactSource(implicit=true)]
// Actual: []
System.out.println("package 'test' types: " +
model.getRootPackage().getPackage("test").getTypes().stream()
.map(t -> t.getSimpleName() + "(implicit=" + t.isImplicit() + ")")
.toList());

// Expected: CompactSource.java -> declaredTypes=[CompactSource(implicit=true)]
// Actual: CompactSource.java -> declaredTypes=[]
for (CtCompilationUnit cu : launcher.getFactory().CompilationUnit().getMap().values()) {
System.out.println(cu.getFile().getName() + " -> declaredTypes=" +
cu.getDeclaredTypes().stream()
.map(t -> t.getSimpleName() + "(implicit=" + t.isImplicit() + ")")
.toList());
}
```

### Actual output

```Java
getAllTypes(): []
package 'test' types: []
CompactSource.java -> declaredTypes=[]
```

### Expected output

```Java
getAllTypes(): [CompactSource(implicit=true)]
package 'test' types: [CompactSource(implicit=true)]
CompactSource.java -> declaredTypes=[CompactSource(implicit=true)]
```

### Spoon Version

11.3.1-beta-8

### JVM Version

JVM Version: 21.0.6

### What operating system are you using?

Alpine Linux 3.19.9