Fix review findings: wishlist qualifying count, improve docs, and lint cleanup

- Count only configs meeting min_count as qualifying in the wishlist
  copy-coverage check (previously ALL guid_to_rid entries inflated the
  denominator, producing phantom wishlist entries), pre-computed once
  per guid instead of per record
- Update stale write_improve_csv doc to match single-best-target
  behavior including cross-copy exclusion
- Borrow owned configs for the trash CSV join instead of cloning the Vec
- Build owned_final_configs via filter_map + flat_map without the
  throwaway intermediate Vec
- Remove runtime assertions on literal constants (clippy
  assertions_on_constants) and replace a redundant closure with the
  function reference (clippy redundant_closure)
- Apply rustfmt across the crate and drop a duplicated test-section header

tests/proptest_tests.rs

@@ -1,7 +1,7 @@
 //! Property-based tests using proptest for sorting and comparison functions.
 
-use std::collections::HashMap;
 use proptest::prelude::*;
+use std::collections::HashMap;
 
 use top_drives::test_utils::{make_car, make_record};
 use top_drives::{compare_cars, compute_car_counts, empty_my_data, AnalysisContext, Car};