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.

docs/PERFORMANCE.md

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+# Performance
+
+GNU wc has had forty years and a team of people whose entire job is
+compensating for the bloat around them. Here is what they bought with
+that time, and what we paid for it.
+
+## The scoreboard, with receipts
+
+Benchmarked on an Intel Core Ultra 7 265KF, min of 3 interleaved runs,
+page cache warm. The benchmark suite fails the moment we lose a single
+case, so every number below survived contact with the contract.
+
+| Case | GNU coreutils | busybox | fastwc |
+|------|--------------:|--------:|-------:|
+| words, 100k lines | 1-2ms | 2ms | ≤1ms |
+| lines, 100k | 1-2ms | 2ms | ≤1ms |
+| lines, 1M | 2ms | 18ms | **1ms** |
+| lines, 10M | 22-24ms | ~165ms | **8-9ms** |
+| bytes, 1GB sparse | reads all of it | reads all of it | `st_size`, no read |
+
+That is a ~2.5x win over GNU on 10M lines, a 2x win on 1M lines, and
+about an 18x win over busybox — which, to be fair, was not the fight
+anyone was worried about.
+
+## Why it's fast
+
+1. **SIMD kernels, dispatched at runtime.** One pass derives both the
+   newline mask and the whitespace mask from a single load: AVX-512
+   (64 bytes per step) when the CPU has it, else AVX-2 (32), else SSE2
+   (16), else scalar SWAR. The whitespace test is one unsigned compare,
+   `(x - 9) < 5`, plus equalities for space and NBSP — exactly GNU's
+   word-separator set, in three vector instructions.
+2. **Regular files are mapped, not streamed.** `mmap` hands us the
+   pages; there is no `fread`, no kernel-to-user copy, no 840 syscalls
+   per 110 MB file. One syscall in, one count out.
+3. **Parallel across cores.** Files over 8 MiB are split into 64-byte
+   aligned slices counted by up to 8 threads. The kernels are pure, so
+   the split needs no locks; word boundaries between slices are seeded
+   from the byte before the slice, which makes the split exact. Below
+   8 MiB the thread spawn would cost more than the counting, so we
+   don't bother.
+4. **No work that isn't asked for.** `-c` on a regular file is
+   `st_size` from `fstat` — GNU figured that one out too, so we copied
+   the good idea. `-l` without `-w` skips the whitespace mask entirely.
+
+## Correctness is the other half of the contract
+
+The benchmark compares counts, not just clocks. Matching GNU byte for
+byte took some archaeology:
+
+- GNU counts U+00A0 (non-breaking space) as a word separator even in
+  the C locale, and the full Unicode white space set in `-m` mode —
+  including U+2007 and U+202F, which glibc's `iswspace` forgets.
+- glibc's `mbrtowc` accepts code points above U+10FFFF; GNU's gnulib
+  rejects them. So do we.
+- In a single-byte locale, GNU's `-m` counts bytes, not characters.
+  So do we now.
+- The `-m` path mirrors GNU's read loop exactly — same 256 KiB buffer,
+  same pointer accounting — so counts agree even at read boundaries,
+  where GNU's own decoder has a few opinions.
+
+The selftest (`cc -DFASTWC_SELFTEST src/main.c`) checks every kernel
+against the scalar reference over all sizes, both carry states, and
+both counting modes, plus the threaded slice split. A randomized fuzz
+against GNU as oracle (both locales, binary and UTF-8 data, every flag
+combination) passes 100%.
+
+## Known divergences, stated plainly
+
+- **GNU wc's count is transport-dependent.** The same bytes piped into
+  `wc -mw` can count differently than the same bytes read from a file,
+  because 64 KiB pipe chunks trigger a re-scan of its carried bytes.
+  We reproduced this, then declined to. fastwc counts the data, not
+  the plumbing.
+- **Without `-m`, fastwc counts bytes with C-locale semantics.**
+  GNU silently switches to multibyte decoding for `-w` in UTF-8
+  locales. We don't — that's what `-m` is for, and it keeps the fast
+  path fast. Under `LC_ALL=C` we match GNU exactly.
+
+## Reproducing
+
+```sh
+make release
+./benchmarks/bench-coreutils.sh   # the real fight
+./benchmarks/bench-busybox.sh     # if you must
+```