feat(FN-4023): preserve triage column when SSE updates contain only priorit

Fix FN-4023: preserve the triage column filter when SSE updates only carry priority changes, preventing unintended column resets during selective priority refreshes. Added regression tests across the task store, useTasks hook, and API routes to cover column-stability scenarios.

Fusion-Task-Id: FN-4023

packages/dashboard/app/hooks/useTasks.ts

@@ -59,7 +59,7 @@ function mergeIncomingTask(current: Task, incoming: Task): Task {
     return { ...incoming, column: current.column, columnMovedAt: current.columnMovedAt };
   }
 
-  if (columnTimestampCompare > 0) {
+  if (columnTimestampCompare >= 0) {
     return { ...incoming, column: current.column, columnMovedAt: current.columnMovedAt };
   }