Add implementation-agnostic PRD for loop workflow example

AS400/CL_examples/Loops/QCLLESRC/prd_spec.md

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+# Product Requirements Document (PRD)
+
+## Title
+Deterministic Loop Workflow and Message Orchestration Service
+
+## Introduction
+This document defines requirements for rebuilding a small workflow service that demonstrates and executes controlled iterative processing with user-visible status messages. The new solution must preserve behavior and intent while remaining implementation-agnostic and suitable for modern platforms.
+
+## Problem Statement
+The current implementation is tightly bound to a specific legacy runtime and is difficult to evolve, test, and integrate. Stakeholders need a clear, language-neutral specification of:
+- What the workflow does
+- How control flow and loop behavior are expected to work
+- How messages are emitted during execution
+- How safety controls prevent non-terminating execution
+
+Without this PRD, teams risk inconsistent reimplementation, missing business behavior, and avoidable runtime defects.
+
+## Objectives
+- Define a canonical behavior contract for loop-driven workflow execution.
+- Preserve deterministic sequence and message outputs from the existing process.
+- Eliminate ambiguous or unsafe loop termination logic.
+- Enable implementation in any modern language/runtime.
+- Provide a foundation for user stories, acceptance tests, and delivery planning.
+
+## Requirements
+### Functional Requirements
+- **FR-1 Workflow Start/Stop:** The system shall initialize execution state and provide explicit completion semantics.
+- **FR-2 Conditional Looping:** The system shall support condition-based loops that execute while a flag/condition evaluates true.
+- **FR-3 Counter Looping:** The system shall support bounded counter loops with configurable start, end, and increment values.
+- **FR-4 Nested Loop Control:** The system shall support nested iterative structures with named control boundaries.
+- **FR-5 Directed Exit:** The system shall allow controlled early exit from nested structures to a defined outer boundary.
+- **FR-6 Message Emission:** The system shall emit human-readable status messages at key points in workflow progression.
+- **FR-7 State Mutation:** The system shall allow runtime mutation of loop-control variables.
+- **FR-8 Termination Safeguards:** The system shall detect and prevent infinite-loop conditions via guardrails (max-iteration, timeout, or explicit termination checks).
+- **FR-9 Observability:** The system shall expose execution trace data (step, loop level, state, and message events) for diagnostics.
+
+### Non-Functional Requirements
+- **NFR-1 Determinism:** Identical inputs and initial state must produce identical outputs and sequence.
+- **NFR-2 Reliability:** Execution must complete cleanly with defined behavior for normal and guarded termination.
+- **NFR-3 Maintainability:** Control-flow logic must be modular and easy to read, test, and modify.
+- **NFR-4 Portability:** No dependency on a specific legacy platform in the functional specification.
+- **NFR-5 Testability:** Behavior must be verifiable through unit, integration, and scenario tests.
+- **NFR-6 Performance:** Typical loop workloads must execute with low latency and predictable resource usage.
+- **NFR-7 Auditability:** Message and trace logs must be retained in a format suitable for troubleshooting.
+
+## User Stories
+- As an **operator**, I want clear execution messages so I can understand workflow progress in real time.
+- As a **developer**, I want configurable loop boundaries so I can adapt behavior without rewriting core logic.
+- As a **maintainer**, I want explicit nested-loop exit controls so I can avoid hidden control-flow defects.
+- As a **quality engineer**, I want deterministic outputs for fixed inputs so I can build reliable automated tests.
+- As a **platform engineer**, I want loop-safety guardrails so the service cannot hang indefinitely in production.
+- As a **product owner**, I want a platform-neutral specification so teams can reimplement consistently in any language.
+
+## Acceptance Criteria
+- A documented execution scenario reproduces the expected sequence: initialization, conditional pass, bounded counter iteration, nested iteration, controlled exit, completion.
+- Message outputs are emitted at each required stage and match defined content/format rules.
+- Counter loop executes exact expected iterations for configured bounds.
+- Nested loop exit behavior routes control to the defined outer boundary and does not continue unintended inner execution.
+- Loop-safety controls trigger under non-terminating conditions and produce a safe, observable failure outcome.
+- Automated tests cover normal flow, boundary values, and guarded-termination paths with passing results.
+- Architecture and behavior documentation are sufficient for independent reimplementation by a separate team.
+
+## Timeline
+- **Week 1:** Requirements finalization, domain modeling, and behavior contract definition.
+- **Week 2:** Architecture design, interfaces, and observability model.
+- **Week 3:** Core workflow engine implementation (conditional/counter loops).
+- **Week 4:** Nested control and directed-exit implementation; safety guardrails.
+- **Week 5:** Test automation, validation, and documentation hardening.
+- **Week 6:** Stakeholder review, release readiness, and handoff.
+
+## Dependencies
+- Product/business stakeholders for behavior confirmation.
+- Engineering team for architecture and implementation.
+- QA team for scenario and regression test coverage.
+- Logging/monitoring stack for execution observability.
+- CI pipeline for automated validation and release checks.
+
+## Risks
+- **Risk:** Misinterpretation of legacy behavior during migration.  
+  **Mitigation:** Use trace-based validation and side-by-side scenario comparison.
+- **Risk:** Infinite-loop or runaway execution in new implementation.  
+  **Mitigation:** Enforce max-iteration/time guards and fail-safe termination paths.
+- **Risk:** Inconsistent message semantics across environments.  
+  **Mitigation:** Define message contract with stable identifiers and formats.
+- **Risk:** Scope drift from demonstration workflow to full orchestration platform.  
+  **Mitigation:** Lock MVP scope and manage expansion through phased backlog.
+- **Risk:** Insufficient test coverage for nested control-flow edge cases.  
+  **Mitigation:** Require boundary/edge-case suites before release approval.
+
+## Conclusion
+This PRD establishes a clear, implementation-neutral specification for rebuilding the loop workflow and messaging process with stronger safety, observability, and maintainability. The next step is to derive detailed user-story breakdowns and testable acceptance scenarios for sprint planning and delivery execution.