FN-5768: add dual-observe workflow parity instrumentation

Add default-off workflow interpreter dual-observe parity instrumentation across core and engine.

- add workflow parity comparison primitives and exports in @fusion/core
- add engine parity observer seam with experimental flag and public exports
- add regression coverage for parity contracts and dual-observe reliability behavior
- document the dual-observe parity mode, events, and contract in workflow docs
- add a patch changeset for @runfusion/fusion release notes

Files changed:
 .changeset/fn-5768-workflow-parity-observer.md     |   9 +
 docs/workflow-steps.md                             |  18 ++
 .../core/src/__tests__/workflow-parity.test.ts     | 143 ++++++++++++++
 packages/core/src/index.ts                         |  18 ++
 packages/core/src/workflow-parity.ts               | 205 +++++++++++++++++++++
 .../workflow-interpreter-dual-observe.test.ts      | 144 +++++++++++++++
 packages/engine/src/index.ts                       |   7 +
 packages/engine/src/workflow-parity-observer.ts    | 116 ++++++++++++
 8 files changed, 660 insertions(+)

Fusion-Task-Id: FN-5768

Fusion-Task-Lineage: fb4f8111-f48d-482c-b1d9-9106c43c829a

packages/engine/src/index.ts

@@ -84,6 +84,13 @@ export {
 export {
   generateSyntheticRunId,
 } from "./run-audit.js";
+export {
+  observeWorkflowParity,
+  WORKFLOW_INTERPRETER_DUAL_OBSERVE_FLAG,
+  type WorkflowParityObserverInput,
+  type WorkflowParityObserverLegacyRunResult,
+  type WorkflowParityObserverShadowRunResult,
+} from "./workflow-parity-observer.js";
 export {
   auditSquashMerge,
   formatSquashAuditReport,