Refactor core APIs, fix bugs, and improve safety

- Make image result/info structs readonly; improve error handling and memory safety in image library
- Introduce IJobScheduler interface; move job scheduling docs to interface
- Remove "index 0 invalid" convention from slot/sparse maps; fix Count logic
- Add Owner<T> for disposable value types in low-level utilities
- Improve ObjectPool<T> thread safety and logic
- Change List<T>.RemoveAndSwapBack to return bool
- Remove unsafe methods from generated math types; add debug range checks
- Update benchmarks and enable collection checks in tests
- Improve documentation, comments, and error messages
- Bump assembly versions across all projects

Misaki.HighPerformance.LowLevel/Collections/UnsafeSparseSet.cs

@@ -85,7 +85,7 @@ public unsafe struct UnsafeSparseSet<T> : IUnsafeCollection<T>
     private int _nextId; // Next available sparse index
     private int _capacity;
 
-    public readonly int Count => _count - 1;
+    public readonly int Count => _count;
     public readonly int Capacity => _capacity;
     public readonly bool IsCreated => _dense.IsCreated && _sparse.IsCreated && _reverse.IsCreated;
 
@@ -133,8 +133,6 @@ public unsafe struct UnsafeSparseSet<T> : IUnsafeCollection<T>
 
         _sparse.AsSpan().Fill(-1);
         _generations.Clear();
-
-        Add(default, out _); // Make index 0 invalid
     }
 
     /// <summary>
@@ -240,8 +238,7 @@ public unsafe struct UnsafeSparseSet<T> : IUnsafeCollection<T>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public readonly bool Contains(int sparseIndex, int generation)
     {
-        // 0 is reserved as invalid index
-        if (sparseIndex <= 0 || sparseIndex >= _sparse.Count)
+        if (sparseIndex < 0 || sparseIndex >= _sparse.Count)
         {
             return false;
         }