FN-6008: document FTS migration defer evidence and align workflow IDs

Refresh the FTS migration guidance with fresh evidence and reconcile workflow interpreter naming.

- document post-FN-5943 production evidence showing the single-file tasks_fts index remains bounded and rebuild-free
- keep the storage guidance explicitly deferred until recurring corruption or sustained bloat is observed
- relax the foreach parser test to accept the current nested loop/foreach error wording
- align the authoritative workflow driver ID with the task branch change during conflict resolution

Files changed:
 docs/storage.md                                         | 7 ++++++-
 packages/core/src/__tests__/workflow-ir-foreach.test.ts | 2 +-
 packages/engine/src/workflow-authoritative-driver.ts    | 2 +-
 3 files changed, 8 insertions(+), 3 deletions(-)

Fusion-Task-Id: FN-6008

Fusion-Task-Lineage: d8c7c05c-f003-453d-bbd1-d9f0dda62f7c

packages/core/src/__tests__/workflow-ir-foreach.test.ts

@@ -115,7 +115,7 @@ describe("foreach validation", () => {
       edges: [] as WorkflowIrEdge[],
     };
     expect(() => parseWorkflowIr(graphWithForeach({ template: tmpl }))).toThrow(
-      /nested foreach/,
+      /nested (loop\/)?foreach/,
     );
   });