Epic: JCL execution — step flow, COND/IF return codes, DD binding, restart/checkpoint #4891

Gap: JCL is scanned (job / step flow, DD names, resolved DSNs are mainframe fact channels) but never executed. A migrated batch job needs: step sequencing, COND= / IF-THEN-ELSE on return codes, PARM passing, DD-to-file binding (incl. concatenations, temporary datasets, SYSOUT), restart from a step, and checkpoint/restart semantics.

Wanted: a JCL execution model that runs the translated programs and the utility runtime (sibling epic) as one job, with condition-code logic proven against IBM's JCL reference; restart/checkpoint behaviour stated or refused by name.

Related: #4508 (JCL harness), #4514 parts 1/4 (data, batch), the batch-utilities epic.

Research basis: 2026-10-11 gap research (runtime inventory, language coverage, issue landscape), saved in the orchestrator handoffs (research-2026-10-11-*). Measure-first: the first child issue of this epic is a reach measurement (how often the class appears and in which constructs), then a design note, then slices that each make programs translate or prove. IBM is the reference, GnuCOBOL / our models the instrument; what IBM leaves open is ASSUMED or REFUSED by name (register).

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