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)

.golangci.toml

 #     "thelper",
 #     "varnamelen",
 # ]
+
+# Benchmark and tool code: unchecked Close/Write/Fprintf is fine.
+[[issues.exclude-rules]]
+path = 'benchmarks/'
+linters = ['errcheck', 'ineffassign']
+
+[[issues.exclude-rules]]
+path = '_bench_test\.go$'
+linters = ['errcheck', 'ineffassign']

benchmarks/doppel_buf_test.go

 		//   n, _ := c.p.writeStream.Read(buf[tls.SizeHeader : tls.SizeHeader+size])
 		//   tls.WriteRecordInPlace(c.Conn, buf[:], n)
 		copy(buf[sizeHeader:], payload)
-		sink = buf[sizeHeader]
 		<-done
+		runtime.KeepAlive(&buf)
 	}()
 
 	// goroutine 2: clock tick loop
 		bp := bufPool.Get().(*[]byte)
 		buf := *bp
 		copy(buf[sizeHeader:], payload)
-		sink = buf[sizeHeader]
 		defer bufPool.Put(bp)
 		<-done
+		runtime.KeepAlive(&buf)
 	}()
 
 	// goroutine 2: clock tick loop
 				defer wg.Done()
 				var buf [maxRecordSize]byte
 				buf[0] = 1
-				sink = buf[0]
 				<-done
+				runtime.KeepAlive(&buf)
 			}()
 		}
 		time.Sleep(200 * time.Millisecond)
 				defer wg.Done()
 				var buf [maxRecordSize]byte
 				copy(buf[sizeHeader:], payload)
-				sink = buf[sizeHeader]
 				<-done
+				runtime.KeepAlive(&buf)
 			}()
 		}
 		time.Sleep(200 * time.Millisecond)
 				bp := bufPool.Get().(*[]byte)
 				buf := *bp
 				copy(buf[sizeHeader:], payload)
-				sink = buf[sizeHeader]
 				defer bufPool.Put(bp)
 				<-done
+				runtime.KeepAlive(&buf)
 			}()
 		}
 		time.Sleep(200 * time.Millisecond)