[Rust][Step] Expose semantic stepping and StepResult

[utilForever]

Parent: #137

Goal

Provide an idiomatic step method over the canonical C step call.

Scope

• Accept the safe Direction enum
• Return safe StepResult and play state values
• Preserve explicit invalid-state errors

Non-goals

• Reward calculation
• Async stepping
• Batch stepping

Acceptance criteria

• One Rust call performs one native semantic step.
• Results match the same C and C++ trace.
• No raw pointer or C status handling leaks into normal callers.

Verification

Replay shared traces through Rust, C, and C++ and compare every step result.