perf(date): wrap stdout in BufWriter to improve batch processing - #9994
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Some CodSpeed benchmarks are showing ~30% performance improvement when system calls are included in the measurement (link): 
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This PR addresses a performance bottleneck when processing large numbers of dates (e.g., using
date -f large_file.txt).Previously, the main loop used
println!, which locks and flushesstdouton every iteration. This change wrapsstdoutin aBufWriterand useswriteln!, significantly improving throughput for batch operations.