fix(bench): stabilize report column/row ordering across input permutations (#39)

- variants: use direct tuple comparison instead of format! string, add sort() after dedup
- Memory section: sort rows by (test_name, backend, variant) before output
- Extra Metrics section: sort rows by (test_name, variant_label) before output
- add [lib] target to Cargo.toml to enable unit tests
- add regression test: same data in different input order produces identical report

crates/bench/Cargo.toml

@@ -3,6 +3,10 @@ name = "log-viewer-bench"
 version = "0.1.0"
 edition = "2024"
 
+[lib]
+name = "log_viewer_bench"
+path = "src/lib.rs"
+
 [[bin]]
 name = "log-viewer-bench"
 path = "src/main.rs"