🧪 Add C# Sphinx integration test

[arnoox]

Closes #77

Summary

C# language support was already fully implemented in the codebase (parser, tree-sitter dependency, scope detection, unit tests, documentation). This PR adds the missing Sphinx integration test to satisfy the last acceptance criterion.

Changes

• tests/doc_test/cs_basic/ — minimal Sphinx project with a .cs source file containing a one-line annotation
• tests/test_src_trace.py — new parametrized case for the cs_basic project in test_build_html
• tests/__snapshots__/test_src_trace/ — generated snapshot asserting IMPL_1 need is emitted from the C# source

Acceptance criteria

• C# parser / comment extractor added to sphinx-codelinks (pre-existing)
• Unit tests covering C# comment annotation parsing (pre-existing)
• Documentation updated to list C# as a supported language (pre-existing)
• Integration test with a minimal C# project ✅ added in this PR

[arnoox]

what is this file for? To be removed?

[arnoox]

• Seems like .. feature:: C# Language Support is missing in features.rst?
• The changelog needs to be updated.