Fix CI: benchmark lint exclusions and data race in doppel test
Benchmarks added in the per-connection-overhead work triggered errcheck
warnings (37 unchecked Close/Fprintf in tool/bench code) and a data race
on package-level `sink` variable written by concurrent goroutines.
- Exclude benchmarks/ and *_bench_test.go from errcheck/ineffassign
- Replace concurrent `sink = buf[X]` with runtime.KeepAlive(&buf)