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/__tests__/builtin-coding-workflow-ir.test.ts

@@ -0,0 +1,34 @@
+import { describe, expect, it } from "vitest";
+import {
+  BUILTIN_CODING_WORKFLOW_IR,
+  WORKFLOW_IR_SCHEMA_VERSION,
+  buildBuiltinCodingWorkflowIr,
+  parseWorkflowIr,
+  serializeWorkflowIr,
+} from "../index.js";
+
+describe("builtin coding workflow ir", () => {
+  it("parses and round-trips", () => {
+    const parsed = parseWorkflowIr(BUILTIN_CODING_WORKFLOW_IR);
+    const reparsed = parseWorkflowIr(serializeWorkflowIr(parsed));
+    expect(reparsed).toEqual(parsed);
+    expect(parsed.schemaVersion).toBe(WORKFLOW_IR_SCHEMA_VERSION);
+  });
+
+  it("contains exactly one start and one end node", () => {
+    const nodes = BUILTIN_CODING_WORKFLOW_IR.nodes;
+    expect(nodes.filter((node) => node.kind === "start")).toHaveLength(1);
+    expect(nodes.filter((node) => node.kind === "end")).toHaveLength(1);
+  });
+
+  it("exposes coding lifecycle stages", () => {
+    const stageNodes = BUILTIN_CODING_WORKFLOW_IR.nodes.filter((node) => node.config?.stage);
+    const stages = stageNodes.map((node) => String(node.config?.stage));
+    expect(stages).toEqual(expect.arrayContaining(["triage", "execute", "review", "merge"]));
+  });
+
+  it("builder returns parser-validated ir", () => {
+    const built = buildBuiltinCodingWorkflowIr();
+    expect(built.metadata.name).toContain("Coding Lifecycle");
+  });
+});