Refactor JobExecutionContext, add scheduler state support

- Refactored JobExecutionContext to use init-only properties and added State and SelfHandle for richer context.
- Updated JobInfo field layout for clarity and memory alignment.
- JobScheduler now accepts and exposes an optional state object, passed to each job context.
- Improved memory management by using _freeList for allocations and disposal.
- WorkerThread and benchmarks updated to use new JobExecutionContext pattern.
- Bumped version to 1.6.1 and performed minor code cleanup.

Misaki.HighPerformance.Test/Benchmark/SPMDBenchmark.cs

@@ -94,7 +94,6 @@ public unsafe class SPMDBenchmark
             height = _SIZE,
         };
 
-        var ctx = new JobExecutionContext(-1, _scheduler);
-        job.Run(_SIZE * _SIZE, in ctx);
+        job.Run(_SIZE * _SIZE, default);
     }
 }