Fix PosWriter off-by-one so each output file holds at most maxPosesPerOutFile positions

[dtran24]

Summary

Fixes an off-by-one error. For example, previously, a file configured with maxPosesPerOutFile = 100000 could contain 100,001 positions. By changing > to >=, this ensures a new file is opened once the configured limit is reached

Testing

Tested locally with a limit of 1 and 3 records:

• Before: one file contained 2 records, another file contained 1 record
• After: each file contained 1 record

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First PR! Wanted to keep it small. Came across the repo recently, and became interested because of things like its distributed self-play approach, recent switch to transformers, and sharing of model internals studies https://github.com/lightvector/katagostudies