fix(csharp): classify interface base types as inherits, not implements

[Synvoya]

Summary

A C# interface's base_list holds base interfaces, so extending another
interface is interface inheritance, not implementation. The C# base_list
emitter in _extract_generic classified every base-list entry with the
name-based class-vs-interface heuristic (_csharp_classify_base), so an
interface extending another interface produced an implements edge.

This diverges from the Java extractor in the same engine, which emits
inherits for extends_interfaces. The result is an inconsistent type
hierarchy: traversing inherits edges misses C# interface hierarchies that
it would capture for Java.

Reproduction (v8 HEAD)

public interface IBase {}
public interface IDerived : IBase {}
public class Impl : IBase {}

inherits:    <none>
implements:  IDerived -> IBase      # wrong: interface extension is inheritance
             Impl     -> IBase      # correct

The equivalent Java (interface IDerived extends IBase) already yields
IDerived --inherits--> IBase.

Fix

When the declaring node is an interface_declaration, classify each base as
inherits (all interface base-list entries are inherited base interfaces).
Class/struct/record declarations keep the existing name-based split — the
first non-interface base stays inherits, interface bases stay implements
— so implements now means only a class/struct/record realizing an
interface, matching the other language extractors.

Base extraction, generic-argument references edges, and the
ref_token/qualified/ref_qualifier metadata used by cross-file
resolution are unchanged; only the emitted relation for interface-declared
bases changes.

Testing

• New regression test test_csharp_interface_extends_is_inherits_not_implements
  covers single- and multi-interface extension and a class implementer as a
  negative control. It fails on v8 HEAD (interface bases land in
  implements) and passes with this change.
• tests/test_languages.py: 315 passed, 13 skipped.
• C# suites test_csharp_type_resolution.py, test_csharp_member_calls.py,
  test_dotnet.py, test_cross_language_call_resolution.py: 89 passed.