test(opy): normalize test ownership around features

[e54-bot]

Summary

• rename Issue-oriented compiler test modules and compatibility fixtures around stable language/compiler/tooling features
• split broad lowering coverage into feature-owned suites and remove the redundant historical control-flow oracle suite
• update fixture provenance, differential expectations, compiler expectations, support matrices, and test references

No compiler semantics or support claims are changed by this reorganization.

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-targets --all-features
• python3 -m unittest discover -s compatibility/tests

Fixes #191