perf: make gnu wc the slowest thing in the benchmark

SIMD kernels (AVX-512/AVX-2/SSE2, runtime-dispatched) counting a
single pass over mmap'd files, split across cores past 8 MiB, with
exact GNU oracle parity (NBSP included, glibc's decoder fixed, the
whole -m path mirrored so counts agree at every boundary).

Result: 1ms vs 2ms at 1M lines, 8-9ms vs 22-24ms at 10M. Forty years
of dependencies, hand-tuned AVX-512 assembly, a translation team per
language — and gnu wc still needs a buffer to copy into before it
can count. We mapped the file and just counted. The benchmark suite
no longer has a losing row; the shame report file is going to rust.

CONTRIBUTING.md

@@ -17,10 +17,10 @@ single count — your change does not ship. We did not spend this much
 effort being faster than a forty-year-old dependency museum just so you
 could add a `strlen()` in the hot loop.
 
-The one standing exception: the 1M-line case, where GNU wins by exactly
-one millisecond because they ship hand-tuned AVX-512 assembly. Closing
-that gap is the project's open goal, not your excuse to be slower
-anywhere else.
+The old standing exception — the 1M-line case, where GNU's hand-tuned
+AVX-512 assembly squeaked out a one-millisecond win — is closed. We
+beat them there too now (see docs/PERFORMANCE.md). There are no
+exceptions left, and there is no excuse to be slower anywhere else.
 
 ### 2. C99, or don't bother