fix: thread-safety in count_sliced, ship measured wins

Review (5-lane) found one MAJOR: count_sliced ignored pthread_create's
return value - on EAGAIN it joined an indeterminate pthread_t and summed
an uninitialized slice, UB plus a silently wrong count. Threads are now
initialized to 0, a failed create counts its slice inline, and nt is
hard-capped at a named MAX_THREADS (the fixed jobs[8]/th[8] arrays
smash the stack past 8 threads - reproduced by QA's thread sweep).

Shipped from the measured optimization hunt:
- -w-only mode skips the newline compare/popcount entirely (the
  (x-9)<5 range already covers '\n' in the whitespace mask) - a
  need_lines gate threads through every kernel, the scalar reference,
  the avx512 mirror, and the sliced workers.
- Thread retune: 12 threads past 32 MiB, 16 past 256 MiB (was 8) -
  up to 18% on 110 MB, ~3% on 1.1 GB in QA's interleaved sweep.
- Benchmark suites pin LC_ALL=C so GNU wc -w can't silently switch to
  multibyte decoding and inflate the win.
- popcount16 is guarded to x86 builds (zero-warnings on other arches);
  checkwc's stray argument dropped.

Verified: selftest (kernels + sliced, now with need_lines coverage),
120-trial fuzz vs GNU (file + stdin, both locales), full test-all suite
three consecutive times, format-check and clang-tidy clean. The 1B
solo monster now lands at 3.7-6s (up to 268 Mlines/s) depending on how
warm the page cache is feeling.

benchmarks/stdin/piping/bench.sh

@@ -14,7 +14,7 @@ REPO_DIR="$(cd "$SCRIPT_DIR/../../.." && pwd)"
 source "$REPO_DIR/benchmarks/std.sh"
 
 checkfastwc
-checkwc coreutils
+checkwc
 BENCH_NAME="coreutils"
 
 printf 'benchmarking %s wc vs fastwc: stdin (%s interleaved runs each, minimum kept)\n' \