Typed compute specifications and the mirai backend

RhoComputeExpressionSpec carries captured R code plus an explicit named argument list. RhoComputeCallSpec carries a worker function plus explicit arguments, avoiding dynamically injected globals in calling packages. rho_submit_compute() is the backend extension point. The convenience functions rho_mirai_eval() and rho_mirai_call() construct the respective specifications and return RhoMiraiTask values.

Details

Worker failures resolve as RhoComputeErrorValue; they are values that downstream packages may translate through S7 dispatch.