#732 Phase 1 Workflow Patterns implemented

Implemented WDP-34/35/36/37 (Task Pre/Post Conditions via DataGuard), WCP-6 (Multi-Choice OR-split), WCP-43 (Explicit Termination). DSL models, builder methods, engine operators, guard evaluation, and tests all complete. 125 DSL tests pass, 27 engine guard tests pass, no regressions.