[stack 38/40] Ratchet accounting mutation evidence - #242

Summary

• strengthen the accounting oracle at epsilon, ordering, argument, and subprocess failure boundaries
• raise the maintained command-runner mutation ratchet from 80% to 90%
• retain a reproducible TAP-runner comparison while rejecting the plugin dependency because subprocess coverage regressed

Measured evidence

• command runner: 208 mutants, 197 killed, 11 diagnostic/equivalent survivors, 0 no-coverage, 0 errors, 94.71%, 125 seconds
• TAP runner trial: 218 mutants, 136 killed, 15 survived, 66 no-coverage, 1 error, 62.67%, 50 seconds
• focused Node suite: 13/13 passed
• docs and Biome checks passed

Stack

Layer 38 of 38. Base: #241. Draft only; do not merge or release until owner approval.

Tracks #196.