feat: Multi-threaded training; play matches every 10 generations

Gradients and losses are computed on rayon's pool in fixed-size chunks
summed in order, so the weights are the same on any number of threads.
Self-play uses the same pool, and the output shows the training time.

The matches against the earlier network and ClaudePlayer now run only
every --match-every generations (default 10), against the generation
that last played them, read back from its saved file.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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