[BUG] SingletonExecutor worker threads never shut down (incomplete #6262 fix)

- Severity: High - Location: `shenyu-disruptor/src/main/java/org/apache/shenyu/disruptor/thread/OrderlyExecutor.java:37,66-77`; `shenyu-disruptor/src/main/java/org/apache/shenyu/disruptor/provider/DisruptorProvider.java:114-121` - Description: In orderly mode `OrderlyExecutor` populates `virtualExecutors` (a `ConcurrentSkipListMap<Long, SingletonExecutor>`) with `corePoolSize` `SingletonExecutor` instances, each its own `ThreadPoolExecutor(1,1,…,new LinkedBlockingQueue<>(), threadFactory)`. `DisruptorProvider.shutdown()` only calls `executor.shutdown()` on the outer `OrderlyExecutor`; `OrderlyExecutor` does not override `shutdown()`/`shutdownNow()` to cascade. The worker thread factory is `DisruptorThreadFactory.create("shenyu_disruptor_consumer_", false)` — `daemon=false` (`DisruptorProviderManage.java:110`) — so the leaked workers are non-daemon and keep the JVM alive. - Impact: After `DisruptorProvider.shutdown()` in orderly mode, `corePoolSize` non-daemon threads (default `availableProcessors()*2`) stay alive forever; JVM cannot exit cleanly. Hot-reload/repeated-init leaks threads each cycle. - Suggested fix: Override `shutdown()`/`shutdownNow()` in `OrderlyExecutor` to iterate `virtualExecutors.values()` and shut down each `SingletonExecutor`, then `super.shutdown()`. - Confidence: High - Related existing: #6262 — but the merged fix only added outer `executor.shutdown()`; inner `SingletonExecutor` leak unfixed. Distinct facet. --- _Identified during the 2026-08-02 deep re-scan; full list in [`docs/scan2-2026-08-02/00-consolidated-critical-high.md`](docs/scan2-2026-08-02/00-consolidated-critical-high.md)._