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/engine/src/workflow-authoritative-driver.ts

@@ -15,7 +15,7 @@ import { executorLog } from "./logger.js";
 import { WORKFLOW_INTERPRETER_DUAL_OBSERVE_FLAG } from "./workflow-parity-observer.js";
 import { WorkflowGraphTaskRunner, type WorkflowGraphTaskRunResult } from "./workflow-graph-task-runner.js";
 
-const AUTHORITATIVE_WORKFLOW_ID = "internal:workflow-interpreter-authoritative";
+const AUTHORITATIVE_WORKFLOW_ID = "workflow-interpreter-authoritative";
 
 export interface WorkflowAuthoritativeDriverStore {
   getSettings(): Promise<Settings>;