Register base classes before derived classes with a topological sort

[cobriensr]

Problem

The entry-point generator orders registered [GodotClass] types so that base classes register before derived classes and this is necessary because registration is eager: GodotRegistry.RegisterClassCore calls classdb_register_extension_class… with the parent's name immediately, so if a class is registered before its base, the engine rejects it (unknown parent) and the whole C# extension fails to load.

But the ordering in EntryPointGenerator was not a topological sort, but a declaration-order-dependent heuristic: it built the type map incrementally inside the same loop and only accumulated a "derived count" for base types already seen, then OrderByDescending'd that count. So the emitted order depends on source/file order.

Concrete failure: three classes Bat : Enemy, Enemy : Mob, Mob : Node declared derived-first (e.g. alphabetical file order) are emitted as Enemy, Mob, Bat -> Enemy before its base Mob -> the engine rejects Enemy -> the extension doesn't load. Nothing about the code is wrong, only the file naming.

Fix

Replace the heuristic with a real topological order: build the complete name -> spec map first, then order by each type's transitive registered-ancestor depth with a stable sort:

int GetRegisteredAncestorDepth(GodotRegistrationSpec spec)
{
    int depth = 0;
    string baseTypeName = spec.FullyQualifiedBaseSymbolName;
    while (specByTypeName.TryGetValue(baseTypeName, out var baseSpec))
    {
        depth++;
        baseTypeName = baseSpec.FullyQualifiedBaseSymbolName;
    }
    return depth;
}
return specs.OrderBy(GetRegisteredAncestorDepth).ToImmutableArray();

A base type always has a strictly lower depth than any type deriving from it, so base classes always come first; OrderBy is stable, so unrelated (equal-depth) classes keep their declaration order. (C# forbids inheritance cycles, so the walk always terminates.)

Verification

• New regression test (EntryPointGeneratorTests): an inheritance chain declared derived-first. Fail-before: the old generator emitted Enemy, Mob, Bat (derived before base). Pass-after: Mob, Enemy, Bat.
• The existing MainWithInheritance snapshot's order changed slightly (a sibling moved so equal-depth types are now in declaration order) — still every base before its derived; regenerated to match.
• Godot.SourceGeneration.Tests: 22/22.

I found this issue while auditing the source generator for adoption-blocking bugs and this issue was not previously filed.