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.LowLevel/Buffer/IAllocator.cs

@@ -143,6 +143,8 @@ public readonly unsafe struct AllocationHandle
     private readonly ReallocFunc _realloc;
     private readonly FreeFunc _free;
 
+    public bool IsValid => _alloc != null && _realloc != null && _free != null;
+
     public AllocationHandle(void* state, AllocFunc alloc, ReallocFunc realloc, FreeFunc free)
     {
         _state = state;