Fix loading configured self-play checkpoints

[bagel786]

Summary

• Load base.load_model_path before training starts.
• Seed the initial self-play pool checkpoint from the loaded policy instead of random initialization.
• Synchronize the asynchronous rollout actor after loading the primary policy.

Fixes #618.

Related: #632 was an earlier closed attempt from another contributor. This PR independently applies the same narrow load-order fix to the current 5c head.

Validation

• git diff --check passed.
• Static inspection verifies that the configured policy load precedes the initial self-play checkpoint save.
• Static inspection verifies that asynchronous actor parameters are copied from the loaded primary policy.
• CUDA build/runtime validation was not available on this machine and should run in CI or on a CUDA host before merge.

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