Enhance JobScheduler and related classes

Added XML documentation comments to the `JobScheduler` class and its methods.
Added a new method `GetJobStatus` in the `JobScheduler` class for job status retrieval.
Added a new `CollectionHandle` struct for collection management.
Added a new test class `TestUnsafeSparseSet` with unit tests for `UnsafeSparseSet`.

Changed the `WorkerThread` class to improve job retrieval logic with a new `FindJob` method.
Changed the `DynamicArena` class by removing commented-out code to streamline memory management.

Removed commented-out code in the `WorkerThread` class for improved readability.
Removed the `ArenaAllocator` struct from `AllocationManager` to clean up unused code.
Removed the `ParallelWriter` struct from `UnsafeSparseSet`, indicating a shift in handling sparse sets.

Misaki.HighPerformance.LowLevel/Collections/CollectionHandle.cs

@@ -0,0 +1,47 @@
+namespace Misaki.HighPerformance.LowLevel.Collections;
+
+public readonly struct CollectionHandle
+{
+    public readonly int id;
+    public readonly int generation;
+
+    public static CollectionHandle Invalid => new(-1, -1);
+
+    public bool IsValid => this != Invalid;
+
+    internal CollectionHandle(int id, int generation)
+    {
+        this.id = id;
+        this.generation = generation;
+    }
+
+    public bool Equals(CollectionHandle other)
+    {
+        return id == other.id && generation == other.generation;
+    }
+
+    public override bool Equals(object? obj)
+    {
+        return obj is CollectionHandle handle && Equals(handle);
+    }
+
+    public override int GetHashCode()
+    {
+        return HashCode.Combine(id, generation);
+    }
+
+    public override string ToString()
+    {
+        return IsValid ? $"CollectionHandle({id}, {generation})" : "CollectionHandle(Invalid)";
+    }
+
+    public static bool operator ==(CollectionHandle left, CollectionHandle right)
+    {
+        return left.Equals(right);
+    }
+
+    public static bool operator !=(CollectionHandle left, CollectionHandle right)
+    {
+        return !(left == right);
+    }
+}