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.Jobs/JobInfo.cs

@@ -84,6 +84,9 @@ internal unsafe struct JobInfo
 
     public const int MAX_DEPENDENTS = 8;
 
+    public void* pJobData;
+    public JobExecutionFunc pExecutionFunc;
+
     // The list of jobs that are waiting for THIS job to complete.
     public fixed int dependentsID[MAX_DEPENDENTS]; // The actual list of IDs
     public fixed int dependentsGeneration[MAX_DEPENDENTS]; // The actual list of generations
@@ -93,9 +96,6 @@ internal unsafe struct JobInfo
 
     public JobRanges jobRanges;
 
-    public void* pJobData;
-    public JobExecutionFunc pExecutionFunc;
-
     public int state;
     public int remainingBatches;