fastwalk: update Darwin values for DefaultNumWorkers

This commit updates DefaultNumWorkers for recent versions of
Darwin/macOS since APFS performance has improved for parallel
access.

```
$ go test -run XXX -bench 'BenchmarkFastWalkNumWorkers' -benchmem -count 3
goos: darwin
goarch: arm64
pkg: github.com/charlievieth/fastwalk
cpu: Apple M4 Pro
BenchmarkFastWalkNumWorkers/NoOp/2-14                 79          14148633 ns/op         2609565 B/op      47022 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/2-14                 80          14094588 ns/op         2609550 B/op      47022 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/2-14                 84          14637930 ns/op         2609919 B/op      47023 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/4-14                120          12065573 ns/op         2605615 B/op      47024 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/4-14                120           9889337 ns/op         2605887 B/op      47024 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/4-14                120          10195627 ns/op         2605582 B/op      47024 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/6-14                100          10942185 ns/op         2609385 B/op      47028 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/6-14                100          10971918 ns/op         2608213 B/op      47028 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/6-14                100          10973288 ns/op         2606743 B/op      47026 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/8-14                100          10754362 ns/op         2611974 B/op      47030 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/8-14                100          10785432 ns/op         2611854 B/op      47029 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/8-14                100          10737051 ns/op         2611918 B/op      47031 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/10-14               100          10858743 ns/op         2613452 B/op      47035 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/10-14               100          10822169 ns/op         2612663 B/op      47035 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/10-14               100          10894342 ns/op         2612256 B/op      47033 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/12-14               100          11385580 ns/op         2612864 B/op      47036 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/12-14               100          11716318 ns/op         2612726 B/op      47035 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/12-14               100          11700127 ns/op         2613368 B/op      47035 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/14-14                88          12586038 ns/op         2612940 B/op      47038 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/14-14                90          12985992 ns/op         2614281 B/op      47037 allocs/op
BenchmarkFastWalkNumWorkers/NoOp/14-14                84          13034864 ns/op         2613378 B/op      47037 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/2-14                 72          16765335 ns/op         4299840 B/op      91473 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/2-14                 63          17172566 ns/op         4299872 B/op      91474 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/2-14                 70          17179273 ns/op         4299852 B/op      91473 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/4-14                 91          12105571 ns/op         4297481 B/op      91489 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/4-14                 93          11821604 ns/op         4297340 B/op      91488 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/4-14                 93          11905724 ns/op         4296724 B/op      91488 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/6-14                100          12188158 ns/op         4300834 B/op      91496 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/6-14                100          12116235 ns/op         4301021 B/op      91498 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/6-14                100          12086306 ns/op         4301788 B/op      91496 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/8-14                100          11655618 ns/op         4306711 B/op      91507 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/8-14                100          11724508 ns/op         4306511 B/op      91507 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/8-14                 98          11728361 ns/op         4306270 B/op      91507 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/10-14               100          11620025 ns/op         4308030 B/op      91515 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/10-14               100          11767371 ns/op         4308027 B/op      91516 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/10-14               100          11654052 ns/op         4307888 B/op      91516 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/12-14                99          12404402 ns/op         4309447 B/op      91526 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/12-14                98          12406126 ns/op         4308881 B/op      91525 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/12-14               100          12399437 ns/op         4309653 B/op      91526 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/14-14                94          13311879 ns/op         4308977 B/op      91524 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/14-14                94          13367483 ns/op         4308668 B/op      91519 allocs/op
BenchmarkFastWalkNumWorkers/NoIO/14-14               100          13301815 ns/op         4308814 B/op      91522 allocs/op
BenchmarkFastWalkNumWorkers/Stat/2-14                 46          25569357 ns/op         8627482 B/op     103532 allocs/op
BenchmarkFastWalkNumWorkers/Stat/2-14                 42          26183286 ns/op         8625687 B/op     103530 allocs/op
BenchmarkFastWalkNumWorkers/Stat/2-14                 45          25584576 ns/op         8626667 B/op     103531 allocs/op
BenchmarkFastWalkNumWorkers/Stat/4-14                 62          17460698 ns/op         8625443 B/op     103535 allocs/op
BenchmarkFastWalkNumWorkers/Stat/4-14                 68          17496477 ns/op         8624933 B/op     103533 allocs/op
BenchmarkFastWalkNumWorkers/Stat/4-14                 62          17613759 ns/op         8626020 B/op     103533 allocs/op
BenchmarkFastWalkNumWorkers/Stat/6-14                 68          16198070 ns/op         8628807 B/op     103536 allocs/op
BenchmarkFastWalkNumWorkers/Stat/6-14                 70          16127585 ns/op         8628775 B/op     103536 allocs/op
BenchmarkFastWalkNumWorkers/Stat/6-14                 68          16261180 ns/op         8628777 B/op     103538 allocs/op
BenchmarkFastWalkNumWorkers/Stat/8-14                 79          14847149 ns/op         8631387 B/op     103540 allocs/op
BenchmarkFastWalkNumWorkers/Stat/8-14                 82          14767673 ns/op         8630988 B/op     103540 allocs/op
BenchmarkFastWalkNumWorkers/Stat/8-14                 79          15044118 ns/op         8630500 B/op     103540 allocs/op
BenchmarkFastWalkNumWorkers/Stat/10-14                74          14559848 ns/op         8634382 B/op     103547 allocs/op
BenchmarkFastWalkNumWorkers/Stat/10-14                74          14777673 ns/op         8633235 B/op     103542 allocs/op
BenchmarkFastWalkNumWorkers/Stat/10-14                78          14525924 ns/op         8632852 B/op     103543 allocs/op
BenchmarkFastWalkNumWorkers/Stat/12-14                76          15086768 ns/op         8634241 B/op     103550 allocs/op
BenchmarkFastWalkNumWorkers/Stat/12-14                76          15245889 ns/op         8633690 B/op     103551 allocs/op
BenchmarkFastWalkNumWorkers/Stat/12-14                75          15269594 ns/op         8633958 B/op     103549 allocs/op
BenchmarkFastWalkNumWorkers/Stat/14-14                73          16315361 ns/op         8635630 B/op     103553 allocs/op
BenchmarkFastWalkNumWorkers/Stat/14-14                73          15708493 ns/op         8635310 B/op     103554 allocs/op
BenchmarkFastWalkNumWorkers/Stat/14-14                70          15530242 ns/op         8634742 B/op     103551 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/2-14              9         118187120 ns/op         5431174 B/op     103543 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/2-14              9         120523607 ns/op         5433512 B/op     103545 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/2-14              9         117043093 ns/op         5434884 B/op     103545 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/4-14             13          92554321 ns/op         5453789 B/op     103557 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/4-14             13          91875324 ns/op         5472566 B/op     103559 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/4-14             12          88067736 ns/op         5473171 B/op     103558 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/6-14             10         106561633 ns/op         5469867 B/op     103568 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/6-14             10         106176221 ns/op         5464262 B/op     103569 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/6-14             10         109333892 ns/op         5460824 B/op     103567 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/8-14              9         129787843 ns/op         5454638 B/op     103575 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/8-14              8         128445740 ns/op         5466150 B/op     103577 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/8-14              8         130875500 ns/op         5482354 B/op     103582 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/10-14             7         171205970 ns/op         5489854 B/op     103596 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/10-14             7         167393381 ns/op         5480689 B/op     103596 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/10-14             6         169856111 ns/op         5473918 B/op     103596 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/12-14             5         208879250 ns/op         5468102 B/op     103600 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/12-14             5         209771892 ns/op         5431465 B/op     103595 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/12-14             5         205633692 ns/op         5441323 B/op     103597 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/14-14             4         255327385 ns/op         5468428 B/op     103601 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/14-14             4         266958646 ns/op         5455972 B/op     103596 allocs/op
BenchmarkFastWalkNumWorkers/ReadFile/14-14             4         265929812 ns/op         5447280 B/op     103599 allocs/op
BenchmarkFastWalkNumWorkers/Hash/2-14                  3         413321292 ns/op        496358816 B/op    146958 allocs/op
BenchmarkFastWalkNumWorkers/Hash/2-14                  3         415285139 ns/op        494946240 B/op    146940 allocs/op
BenchmarkFastWalkNumWorkers/Hash/2-14                  3         417991528 ns/op        494889464 B/op    146958 allocs/op
BenchmarkFastWalkNumWorkers/Hash/4-14                  4         284903771 ns/op        523706368 B/op    147738 allocs/op
BenchmarkFastWalkNumWorkers/Hash/4-14                  4         263967323 ns/op        522278086 B/op    147719 allocs/op
BenchmarkFastWalkNumWorkers/Hash/4-14                  4         319274938 ns/op        535807092 B/op    148080 allocs/op
BenchmarkFastWalkNumWorkers/Hash/6-14                  5         263477250 ns/op        541640248 B/op    148240 allocs/op
BenchmarkFastWalkNumWorkers/Hash/6-14                  4         258025010 ns/op        537553382 B/op    148147 allocs/op
BenchmarkFastWalkNumWorkers/Hash/6-14                  4         288945302 ns/op        537808348 B/op    148192 allocs/op
BenchmarkFastWalkNumWorkers/Hash/8-14                  4         272007062 ns/op        538232894 B/op    148329 allocs/op
BenchmarkFastWalkNumWorkers/Hash/8-14                  4         256360417 ns/op        536005406 B/op    148201 allocs/op
BenchmarkFastWalkNumWorkers/Hash/8-14                  4         260579344 ns/op        537674906 B/op    148233 allocs/op
BenchmarkFastWalkNumWorkers/Hash/10-14                 4         264377073 ns/op        536361002 B/op    148330 allocs/op
BenchmarkFastWalkNumWorkers/Hash/10-14                 4         262230896 ns/op        540794086 B/op    148423 allocs/op
BenchmarkFastWalkNumWorkers/Hash/10-14                 5         273237033 ns/op        536818494 B/op    148359 allocs/op
BenchmarkFastWalkNumWorkers/Hash/12-14                 5         244473758 ns/op        535679438 B/op    148404 allocs/op
BenchmarkFastWalkNumWorkers/Hash/12-14                 4         256615219 ns/op        537811416 B/op    148468 allocs/op
BenchmarkFastWalkNumWorkers/Hash/12-14                 5         300437217 ns/op        537495635 B/op    148426 allocs/op
BenchmarkFastWalkNumWorkers/Hash/14-14                 5         263509267 ns/op        536052094 B/op    148580 allocs/op
BenchmarkFastWalkNumWorkers/Hash/14-14                 4         275321542 ns/op        532870428 B/op    148504 allocs/op
BenchmarkFastWalkNumWorkers/Hash/14-14                 4         334394281 ns/op        550532966 B/op    148904 allocs/op
PASS
ok      github.com/charlievieth/fastwalk        156.078s
```

ci: test on go1.24

gh: add code coverage

gh: add code coverage

gh: add code coverage