Port movement scratch initialization and callback plans

RE-985. Focused validation: cargo xtask test-group rust-client-move-scratch. Machine report.

Rust owns an exact268-byte scratch slice and returns ordered zero/mode-callback plans. Twelve source/Rust fixtures compare every scratch word, guards and callback order. Invalid client slots reject before mutation; slice ownership prevents overwriting neighboring frame words. Callback execution and feedback are deliberately left to a future native adapter.