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/JobExecutionContext.cs

@@ -4,17 +4,21 @@ public readonly ref struct JobExecutionContext
 {
     public int ThreadIndex
     {
-        get;
+        get; init;
     }
 
     public IJobScheduler JobScheduler
     {
-        get;
+        get; init;
     }
 
-    public JobExecutionContext(int threadIndex, IJobScheduler jobScheduler)
+    public object? State
     {
-        ThreadIndex = threadIndex;
-        JobScheduler = jobScheduler;
+        get; init;
+    }
+
+    public JobHandle SelfHandle
+    {
+        get; init;
     }
 }