RustLean v4: Rust FIM Completion

Fill-in-the-middle completion from AfkaraLP/rustlean-gguf (Qwen2.5-Coder-1.5B, Q8_0). The v4 release improves repository-disjoint Rust completion while retaining the base model's compiler-tested pass rate. Put <|cursor|> where you want the model to write code. Everything before it is the prefix, everything after is the suffix.