Initial commit: E4B-MarkBase model integration with passing tests

- E4B-MarkBase model (42 layers, 4.4GB) loaded successfully
- All Phase 1-6 tests passed (model loading, forward pass, vision/audio towers, token generation, performance)
- All stress tests passed (5/5 in 127.6s)
  - Concurrent inference
  - Memory stress (67.5 tok/s, 0 NaN)
  - Continuous generation
  - Batch processing
  - Long-running stability
- Swift Metal inference engine with multimodal support

Sources/MarkBase/Weights/TensorDescriptor.swift

@@ -0,0 +1,38 @@
+/// Metadata for a single tensor stored in a SafeTensors file.
+public struct TensorDescriptor: Sendable, Codable {
+    public let name: String
+    public let dtype: TensorDType
+    public let shape: [Int]
+    /// Byte offset from the start of the safetensors data section.
+    public let dataOffset: Int
+    /// Byte size of the tensor data.
+    public let dataSize: Int
+
+    /// Total number of elements.
+    public var elementCount: Int { shape.reduce(1, *) }
+
+    /// Check if shape is compatible with a given dim count.
+    public func hasRank(_ rank: Int) -> Bool { shape.count == rank }
+
+    /// For quantized tensors: returns the grouping factor (elements per group).
+    /// MLX default: 64 elements per quantization group (for Gemma 4 E4B 4-bit).
+    public var quantizationGroupSize: Int { 64 }
+}
+
+/// Group of tensors that together represent a quantized linear layer.
+///   weight: U32 packed (shape: [outDim, inDim / 32 * 4])
+///   scales: BF16    (shape: [outDim, inDim / 32])
+///   biases: BF16    (shape: [outDim, inDim / 32])
+public struct QuantizedTensorGroup: Sendable {
+    public let name: String
+    public let weight: TensorDescriptor
+    public let scales: TensorDescriptor
+    public let biases: TensorDescriptor
+
+    /// Output dimension.
+    public var outDim: Int { weight.shape[0] }
+    /// Input dimension (pre-quantization).
+    public var inDim: Int { scales.shape[1] * 32 }
+    /// Block size (elements per group).
+    public let groupSize: Int = 64
+}