Fix position counter in data generation

[Copilot]

Position counter was initialized to DATAGEN_NUM_RAND instead of 0, inflating reported position counts by the number of random opening moves (typically 12 per game). These opening moves are never written to the output PGN.

Changes

• engine/main.cpp:70: Initialize plies to 0 instead of DATAGEN_NUM_RAND

// Before
int plies = DATAGEN_NUM_RAND;

// After  
int plies = 0;

This also corrects draw adjudication to trigger at 80 moves in the recorded game rather than 68.

Original prompt

Check over the files engine/main.cpp and runner.py for mistakes in the internal data generation process.

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