feat: add turn_canceled event for proper interruption handling

Adds a new turn_canceled event type to distinguish intentional turn
interruption from errors. Fixes race conditions in Claude agent where
concurrent stream() calls could cause garbled output by serializing
turn handoff with turn IDs.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/app/src/types/stream.ts

@@ -819,6 +819,7 @@ export function reduceStreamUpdate(
     case "turn_started":
     case "turn_completed":
     case "turn_failed":
+    case "turn_canceled":
     case "permission_requested":
     case "permission_resolved":
     case "attention_required":