bench: report fastwc timing in microseconds

Sub-millisecond runs showed as a flat 0ms, hiding real differences. The
suite now times fastwc with a time_us helper, keeps the minimum reading
in µs, derives ms from it, and prints both (fastwc: 0ms (767µs)). GNU wc
stays at ms precision; solo case shows µs too.

benchmarks/stdin/piping/bench.sh

@@ -19,7 +19,7 @@ BENCH_NAME="coreutils"
 
 printf 'benchmarking %s wc vs fastwc: stdin (%s interleaved runs each, minimum kept)\n' \
     "$BENCH_NAME" "$BENCH_REPS"
-printf '%-28s %10s %10s %8s %s\n' 'test' 'wc' 'fastwc' 'ratio' 'status'
+printf '%-28s %10s %22s %8s %s\n' 'test' 'wc' 'fastwc (ms µs)' 'ratio' 'status'
 
 printf '%s\n' '--- stdin lines ---'
 if ! run_stdin_cases lines -l 10000 100000 1000000 10000000; then