SIMD Refactor: Merge simd-dev into dev (#55)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: aider (openrouter/anthropic/claude-sonnet-4) <aider@aider.chat>
Co-authored-by: Warp <agent@warp.dev>

lib/core/simd/SimdExtensions.md

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+# SimdExtensions Class
+
+`SimdExtensions` provides high-performance, SIMD-accelerated extension methods for `ReadOnlySpan<double>`. It leverages .NET's `Vector<T>` to achieve 4-8x speedups on supported hardware (AVX2, AVX-512) while automatically falling back to scalar implementations on older hardware.
+
+## Key Features
+
+* **Hardware Acceleration**: Uses CPU vector registers to process multiple elements in parallel.
+* **Automatic Fallback**: Gracefully handles non-SIMD hardware or small arrays.
+* **Zero-Allocation**: Operates directly on spans without creating new arrays.
+* **Aggressive Inlining**: Methods are marked for inlining to minimize call overhead.
+
+## Available Methods
+
+| Method | Description |
+| ------ | ------ |
+| `ContainsNonFinite()` | Checks if span contains any non-finite values (NaN or Infinity). |
+| `SumSIMD()` | Calculates the sum of elements. |
+| `MinSIMD()` | Finds the minimum value. |
+| `MaxSIMD()` | Finds the maximum value. |
+| `MinMaxSIMD()` | Finds both min and max in a single pass (more efficient than separate calls). |
+| `AverageSIMD()` | Calculates the arithmetic mean. |
+| `VarianceSIMD()` | Calculates the sample variance. |
+| `StdDevSIMD()` | Calculates the sample standard deviation. |
+| `DotProduct()` | Calculates the dot product of two spans. |
+
+## Performance
+
+On modern CPUs (e.g., Intel Core i7/i9, AMD Ryzen), these methods typically outperform standard LINQ or scalar loops by a factor of 4 to 8 for large arrays.
+
+## Usage
+
+```csharp
+using QuanTAlib;
+
+double[] data = { 1.0, 2.0, 3.0, 4.0, 5.0, ... };
+ReadOnlySpan<double> span = data;
+
+// Calculate sum
+double sum = span.SumSIMD();
+
+// Calculate min and max in one pass
+var (min, max) = span.MinMaxSIMD();
+
+// Calculate standard deviation
+double stdDev = span.StdDevSIMD();
+
+// Check for valid data
+bool hasInvalid = span.ContainsNonFinite();
+
+// Calculate dot product
+double dot = span.DotProduct(otherSpan);
+```