Migrate XPOIS onto the shared worker executors #59

PR #54 adds shared Dragon and MPI executors while XPOIS retains its own coordinators, receipt validation, and lifecycle audits. Carmelo requested this follow-up in his review.

Migrate XPOIS work onto WorkloadSpec and the shared worker protocol, then remove the duplicate coordinators. Preserve the existing CLI, repeated-round behavior, input identity checks, failure artifacts, and cleanup guarantees. Qualify the migrated path with CPU failure tests and real multi-rank MPI/Dragon runs before replacing the existing implementation.

This is separate from the focused review fixes in #50, #54, and #56.