Refactor SPMD job system, add GGX mipmap benchmark

- Replace IJobSPMD with T4-generated, multi-type SPMD job interfaces and wrappers (up to 8 numeric types)
- Extend ISPMD with Cast/BitCast; implement for ScalarLane and WideLane (SIMD-aware)
- Add unary minus, scalar-lane, and lane-scalar operators to Vector2/3/4; improve Select methods
- WideLane now partial with T4-generated Cast/BitCast (SIMD conversions)
- SPMD job Execute now requires unmanaged TLane; update all usages and benchmarks
- Add GGXMipGenerationBenchmark with vectorized and scalar paths, SkiaSharp output
- Update project files: add generated code, SkiaSharp, bump version to 1.3.0
- Misc: fix formatting, method signatures, FreeList logic

Misaki.HighPerformance.Mathematics.SPMD/ScalerLane.cs

@@ -123,6 +123,22 @@ public readonly unsafe struct ScalarLane<TNumber> : ISPMD<ScalarLane<TNumber>, T
         return Vector.Create(value);
     }
 
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public TOther Cast<TOther, TOtherNumber>()
+        where TOther : ISPMD<TOther, TOtherNumber>
+        where TOtherNumber : unmanaged, INumber<TOtherNumber>, IBinaryNumber<TOtherNumber>, IMinMaxValue<TOtherNumber>, IBitwiseOperators<TOtherNumber, TOtherNumber, TOtherNumber>
+    {
+        return TOther.Create(TOtherNumber.CreateChecked(value));
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public TOther BitCast<TOther, TOtherNumber>()
+        where TOther : ISPMD<TOther, TOtherNumber>
+        where TOtherNumber : unmanaged, INumber<TOtherNumber>, IBinaryNumber<TOtherNumber>, IMinMaxValue<TOtherNumber>, IBitwiseOperators<TOtherNumber, TOtherNumber, TOtherNumber>
+    {
+        return Unsafe.BitCast<ScalarLane<TNumber>, TOther>(this);
+    }
+
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static ScalarLane<TNumber> operator +(ScalarLane<TNumber> a, ScalarLane<TNumber> b)
     {