feat: add workflow stage resume (#747)

* feat: add workflow stage resume

* test: cover workflow resume output processors

* fix: harden workflow resume metadata

* fix: address workflow resume review feedback

* feat: add workflow review gate controls

* fix: validate workflow metadata shape

* fix: validate workflow rerun inputs

Author: andreatgretel

fern/versions/latest/pages/concepts/workflow-chaining.mdx

@@ -102,10 +102,38 @@ workflow.add_stage("cleanup", cleanup)
 
 This is useful for final cleanup, schema transforms, and format-specific export preparation.
 
+## Resume
+
+Workflow names are durable artifact identities. Reusing the same name with `resume=ResumeMode.IF_POSSIBLE` reuses compatible completed stages, resumes a matching partial stage through `DataDesigner.create(..., resume=ResumeMode.ALWAYS)`, and reruns the first changed or missing stage plus its descendants.
+
+```python
+from data_designer.interface import ResumeMode
+
+results = workflow.run(resume=ResumeMode.IF_POSSIBLE)
+```
+
+Use `ResumeMode.ALWAYS` for strict resume before the first recovered checkpoint. A changed stage or missing selected output raises instead of starting fresh. If a matching partial stage resumes successfully, descendants are recreated from that stage's current output.
+
+## Review gates
+
+Use `targets` to materialize an intermediate stage without running the rest of the workflow. `export_stage()` writes the selected stage output for review. After review, pass the approved parquet as a stage output override and resume the downstream target.
+
+```python
+draft_results = workflow.run(targets="drafts")
+draft_results.export_stage("drafts", "drafts_for_review.parquet")
+
+results = workflow.run(
+    targets="expanded",
+    resume=ResumeMode.IF_POSSIBLE,
+    stage_output_overrides={"drafts": "approved.parquet"},
+)
+```
+
+If the reviewed data replaces a stage's selected output in place, run with `resume=ResumeMode.IF_POSSIBLE` and `rerun_from="expanded"` to rebuild that stage and its descendants from the current boundary output.
+
 ## Current limits
 
 - Stages are linear. DAGs, parallel branches, and joins are planned separately.
-- Stage-level resume is not implemented yet.
 - `push_to_hub()` does not support selected processor or callback outputs yet. Use `export()` for the selected workflow output.
 - `on_success` callbacks are trusted user code. If a callback returns a path, Data Designer reads that path as the next stage input.
 - The artifact layout is intended for inspection, but it is not yet a stable public contract.