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/UnitTest/Jobs/TestJobSystem.cs

@@ -275,8 +275,7 @@ public unsafe class TestJobSystem
             height = size,
         };
 
-        var ctx = new JobExecutionContext(-1, s_jobScheduler);
-        vectorJob.Run(size * size, in ctx);
+        vectorJob.Run(size * size, default);
 
         var spmdBuf = stackalloc float[size * size];
         var ss = new Span<float>(spmdBuf, size * size);