style: fix selector generator Clippy warnings and formatting

[egorsmkv]

Fix the existing warnings that blocked strict Clippy and rustfmt after the benchmark harness change. Collapse two nested attribute-selection conditions and use stable sort_by_key with Reverse(score) for descending predicate ranking, preserving tie ordering and behavior. Apply rustfmt to selector_generator.rs; no dependency or API changes.

Validation:

• cargo clippy --locked --all-targets -- -D warnings passes without lint exceptions.
• cargo fmt --all --check and git diff --check pass.
• 31 Rust unit tests and 2 benchmark fixture integration tests pass.
• All 82 Python tests pass against an extension rebuilt from this branch (dev profile, CPython 3.14.7).

Only src/selector_generator.rs changes.