Address fourth review round: observability, docs accuracy, dep hygiene, tests

- Warn when no timing data was found at all (empty --times-dir previously
  produced empty CSVs without any hint) — best_cars empty + non-empty car
  list now logs a warning in parse_all_times
- Remove the inaccurate "known gap" doc from write_trash_csv: category 1
  (not best on any track) already covers non-final cars, so the
  documented gap never existed
- Gate env_logger behind the bin-logging feature (default-on) and mark
  the records binary as requiring it, so downstream lib consumers do not
  pull env_logger transitively; lib builds clean with --no-default-features
- Switch fetch_car progress output from println! to log::info! so
  RUST_LOG controls it
- atomic_write takes &Path instead of &PathBuf
- Add direct unit tests for is_final_build (sum-8 finals, maxed builds,
  sub-8 non-finals, malformed inputs) and a parsing test covering
  missing/null/non-numeric "t" fields (warn + skip, valid entries still
  parsed)

Cargo.toml

@@ -7,9 +7,12 @@ edition = "2021"
 name = "top_drives"
 path = "src/lib.rs"
 
+# Bin-only (logger init in records.rs); required-features keeps the lib crate's
+# downstream consumers free of env_logger unless they opt in.
 [[bin]]
 name = "records"
 path = "src/records.rs"
+required-features = ["bin-logging"]
 
 [dependencies]
 reqwest = "0.12.15"
@@ -21,10 +24,11 @@ clap = { version = "4", features = ["derive"] }
 thiserror = "2"
 dashmap = "6"
 log = "0.4"
-env_logger = "0.11"
+env_logger = { version = "0.11", optional = true }
 
 [features]
-default = []
+default = ["bin-logging", "test-utils"]
+bin-logging = ["dep:env_logger"]
 test-utils = []
 
 [dev-dependencies]