FN-5766: add flagged-off workflow graph executor scaffold

Add the Phase 2 workflow-graph interpreter scaffold and builtin coding IR wiring while keeping execution behavior unchanged by default.

- add BUILTIN_CODING_WORKFLOW_IR and builder in @fusion/core with coverage tests
- export new coding workflow IR APIs from core index
- add WorkflowGraphExecutor scaffold in @fusion/engine plus parity-focused test coverage
- document the flagged-off interpreter scaffold, parity gate, and v1 IR gap reconciliation in workflow docs
- include a changeset for published @runfusion/fusion

Files changed:
 .../fn-5766-workflow-graph-executor-scaffold.md    | 11 +++
 docs/workflow-steps.md                             | 27 +++++++
 .../__tests__/builtin-coding-workflow-ir.test.ts   | 34 +++++++++
 packages/core/src/builtin-coding-workflow-ir.ts    | 60 ++++++++++++++++
 packages/core/src/index.ts                         |  4 ++
 .../workflow-graph-executor-parity.test.ts         | 32 +++++++++
 packages/engine/src/index.ts                       |  7 ++
 packages/engine/src/workflow-graph-executor.ts     | 82 ++++++++++++++++++++++
 8 files changed, 257 insertions(+)

Fusion-Task-Id: FN-5766
Fusion-Task-Lineage: 4bbef713-a415-4d42-a20b-9ae4f3235419

packages/core/src/index.ts

@@ -16,6 +16,10 @@ export {
   WorkflowIrError,
   BUILTIN_WORKFLOW_IR_FIXTURE,
 } from "./workflow-ir.js";
+export {
+  BUILTIN_CODING_WORKFLOW_IR,
+  buildBuiltinCodingWorkflowIr,
+} from "./builtin-coding-workflow-ir.js";
 export { customProviderRegistryKey } from "./custom-provider-key.js";
 export { MOCK_PROVIDER_ID } from "./mock-provider-constants.js";
 export type { MockProviderId, MockSessionPurpose } from "./mock-provider-constants.js";