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

.changeset/fn-5766-workflow-graph-executor-scaffold.md

@@ -0,0 +1,11 @@
+---
+"@runfusion/fusion": minor
+---
+
+Add a flagged-off Workflow Graph Executor scaffold and built-in coding lifecycle Workflow IR exports.
+
+- Adds `BUILTIN_CODING_WORKFLOW_IR` and `buildBuiltinCodingWorkflowIr` to `@fusion/core`.
+- Adds `WorkflowGraphExecutor` and `WORKFLOW_GRAPH_EXECUTOR_FLAG` to `@fusion/engine`.
+- Adds parity-harness skeleton tests and IR documentation updates.
+
+The new executor path is gated by `experimentalFeatures.workflowGraphExecutor` and remains strict no-op while disabled (default).