Improve spin-wait, SPMCQueue, and add UnsafeString/Text

- JobScheduler: dynamic spin-wait thresholds for lower latency; removed SLEEP_THRESHOLD constant.
- SPMCQueue: switched to explicit struct layout, removed manual padding, fixed power-of-two capacity bug.
- WorkerThread: enhanced work-stealing with additional cascade loop.
- AllocationHandle: added IsValid property.
- VirtualMemoryBlock: fixed Dispose to pass correct size to Munmap.
- Added UnsafeString and UnsafeText for zero-allocation string/text handling.
- Updated project metadata, versions, and repository URLs.
- Minor inlining/optimization tweaks in GGXMipGenerationBenchmark.

src/Misaki.HighPerformance.Jobs/WorkerThread.cs

@@ -84,6 +84,11 @@ internal class WorkerThread : IDisposable
             {
                 return true;
             }
+        }
+
+        for (var offset = 0; offset < helperThreadCount; offset++)
+        {
+            var p = cascade[index + offset];
 
             for (var i = 1; i < _scheduler.WorkerCount; i++)
             {