Refactor karva_test_semantic for readability and discoverability

[MatthewMckee4]

Summary

Readability refactors across karva_test_semantic, no behaviour change. classify_test_result loses its eleven-argument signature and three-level nested match in favour of a small VariantReportCtx struct with early returns. The functions/ module now follows the same mod.rs barrel + python.rs PyO3 surface convention as tags/ and fixtures/. Python interpreter attachment moves out of utils.rs into a new src/py_attach.rs with attach_with_output replacing the misnamed attach_with_project.

Test plan

ci

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 1 untouched benchmark

---

_{Comparing refactor/test-semantic-cleanup (87097fe) with main (2f2b75d)}