RL Artifact Paths

Trainers and editor-side viewers share the data/rl/{run}_{artifact}.json convention. The run name is explicit so another example can reuse it without adding a filename declaration.

def Runtime.RL.Artifacts.path (run : String) (artifact : String := "trainlog") : System.FilePath

JSON artifact path for a named run, defaulting to its training log.

For example, path "ppo_gridworld" "policy" names the saved policy snapshot. Arguments are filename components supplied by the caller, not sanitized user input. CLI flags can override the resulting default path.