fix(csharp): emit INHERITS edges from base_list clause

[Saeed-Nourizadeh]

Problem

C# classes produce zero INHERITS edges. _get_bases() handles C# in the shared java/kotlin/csharp branch, which only looks for superclass / super_interfaces / delegation_specifier nodes. The tree-sitter-csharp grammar emits none of those — it wraps a class's : Base, IFace clause in a single base_list node. So the branch never matches and C# inheritance is lost: inheritors_of / get_impact_radius return empty for .cs files.

Reproduced on a real 1,527-file / 440K-LoC C# codebase (SplendidCRM): 0 INHERITS edges before, 1,373 after this fix (top bases that were previously invisible: framework base classes like SplendidControl with 440 subclasses, page bases, System.Web.Services.WebService, etc.).

Fix

Give C# its own branch in _get_bases() that drills into base_list for the named base-type children — identifier, qualified_name, generic_name — and stores the bare type name (skipping the leading : and commas). Kotlin keeps its existing handling; Java is unaffected. This mirrors the approach in #278 (Java) and the C++ base_class_clause branch.

public class CachedRepo : InMemoryRepo, IRepository   ->  INHERITS: InMemoryRepo, IRepository
public class DisposableService : System.IDisposable   ->  INHERITS: System.IDisposable  (qualified_name preserved)
public class UserList : List<User>                     ->  INHERITS: List<User>          (generic_name preserved)

Tests

• Extends tests/fixtures/Sample.cs with extends + implements, a qualified base (System.IDisposable) and a generic base (List<User>).
• Adds test_finds_inheritance and test_inheritance_target_is_bare_name to TestCSharpParsing (asserts targets are bare type names — no leading :, no comma-joined clauses).
• pytest tests/test_multilang.py -k "CSharp or Java or Kotlin" → 23 passed (no regression). ruff check clean.