🧪 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]

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

[arnoox]

Pushed fixes for both findings:

• Added the missing C# Language Support .. feature:: entry to features.rst (parser was implemented but never documented — it now matches the format of the C/C++/Python/YAML/Rust entries, with FE_CSHARP, FAULT_CSHARP_1, FAULT_CSHARP_2).
• Added changelog entries under "Upcoming" for the docs update and the new C# integration test.

Re the doctree.xml snapshot file: replied inline — it's the generated syrupy snapshot for the new cs_basic test case and is required, not stray.

Verified tox -e docs-update (-nW --keep-going) builds clean with the new feature/fault IDs, and tox -e py312-sphinx8-needs5 -- tests/test_src_trace.py passes with all 5 snapshots.