[Perf][Qwen3-TTS] Enable event-driven orchestration by default (#7088)

Signed-off-by: Sy03 <1370724210@qq.com>

docs/design/module/engine_orchestration.md

@@ -79,7 +79,7 @@ the boundary affected by the in-flight stage client/process refactor in
 responsibilities proposed only by that PR are not current contracts.
 
 The orchestration loop also has an opt-in event-driven mode
-(`VLLM_OMNI_EVENT_DRIVEN_ORCH=1`, default off) proposed in
+(`VLLM_OMNI_EVENT_DRIVEN_ORCH=1`, default off except for Qwen3-TTS) proposed in
 [#5221](https://github.com/vllm-project/vllm-omni/pull/5221). It changes poll
 cadence only: the routing, ordering, and terminal-state contracts below hold
 identically on both loops.