dsh-fork-perf: DSH Conversation Fork Performance Optimization Plugin

The `dsh-fork-perf` plugin for DeepSeek Harness (DSH) is designed to address performance bottlenecks when forking large sessions, reducing the time taken by approximately 1/18. It reuses references to frozen JSON trees through a "zero-copy fork" and optimizes the initialization process of `PersistenceCoordinator.initFor`. The plugin provides an HTTP API for monitoring and configuration, along with robust fallback mechanisms and patch restoration capabilities. Note that it depends on DSH version 0.1.0-rc.6, and is limited by the fact that copies inside the `enqueue` closure cannot be eliminated.