cksum: Move default stdin to clap

[oech3]

Ref #9992

[codspeed-hq]

CodSpeed Performance Report

Merging #10042 will degrade performance by 6.79%

_{Comparing oech3:cksum-stdin-clap (0f202ca) with main (8cf88cd)}

Summary

❌ 1 regression
✅ 138 untouched
⏩ 37 skipped¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Efficiency
❌	cksum_blake3	209.7 µs	225 µs	-6.79%

Footnotes

1. 37 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩