chore: import upstream snapshot with attribution

.claude/rules/legacy-v3-code.md

@@ -0,0 +1,33 @@
+---
+paths:
+  - "apps/webapp/app/v3/**"
+---
+
+# Legacy V1 Engine Code in `app/v3/`
+
+The `v3/` directory name is misleading - most code here is actively used by the current V2 engine. Only the specific files below are legacy V1-only code.
+
+## V1-Only Files - Never Modify
+
+- `marqs/` directory (entire MarQS queue system: sharedQueueConsumer, devQueueConsumer, fairDequeuingStrategy, devPubSub)
+- `legacyRunEngineWorker.server.ts` (V1 background job worker)
+- `services/triggerTaskV1.server.ts` (deprecated V1 task triggering)
+- `services/cancelTaskRunV1.server.ts` (deprecated V1 cancellation)
+- `authenticatedSocketConnection.server.ts` (V1 dev WebSocket using DevQueueConsumer)
+- `sharedSocketConnection.ts` (V1 shared queue socket using SharedQueueConsumer)
+
+## V1/V2 Branching Pattern
+
+Some services act as routers that branch on `RunEngineVersion`:
+- `services/cancelTaskRun.server.ts` - calls V1 service or `engine.cancelRun()` for V2
+- `services/batchTriggerV3.server.ts` - uses marqs for V1 path, run-engine for V2
+
+When editing these shared services, only modify V2 code paths.
+
+## V2 Modern Stack
+
+- **Run lifecycle**: `@internal/run-engine` (internal-packages/run-engine)
+- **Background jobs**: `@trigger.dev/redis-worker` (not graphile-worker/zodworker)
+- **Queue operations**: RunQueue inside run-engine (not MarQS)
+- **V2 engine singleton**: `runEngine.server.ts`, `runEngineHandlers.server.ts`
+- **V2 workers**: `commonWorker.server.ts`, `alertsWorker.server.ts`, `batchTriggerWorker.server.ts`