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BPE cache: per-thread read-through cache to avoid RwLock atomics on hits

BPE cache: per-thread read-through cache to avoid RwLock atomics on hits: a task in repo2rlenv-pr-diff (Harbor dataset). Title: BPE cache: per-thread read-through cache to avoid RwLock atomics on hits

The task

**Title:** BPE cache: per-thread read-through cache to avoid RwLock atomics on hits

Part of FineEnvs/repo2rlenv-pr-diff.