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/SafeTensorsIndex.swift

@@ -0,0 +1,34 @@
+import Foundation
+
+/// Handles sharded SafeTensors models (with model.safetensors.index.json).
+public final class SafeTensorsIndex {
+    public let weightMap: [String: String]
+    public let baseDir: String
+
+    /// Load the index file from a model directory.
+    public init(modelDir: String) throws {
+        let indexURL = URL(fileURLWithPath: modelDir).appendingPathComponent("model.safetensors.index.json")
+        let data = try Data(contentsOf: indexURL)
+        guard let json = try JSONSerialization.jsonObject(with: data) as? [String: Any],
+              let weightMap = json["weight_map"] as? [String: String]
+        else {
+            throw WeightError.invalidHeader("Index file missing weight_map")
+        }
+        self.weightMap = weightMap
+        self.baseDir = modelDir
+    }
+
+    /// All unique shard filenames referenced by the index.
+    public var shardFiles: Set<String> {
+        Set(weightMap.values)
+    }
+
+    /// Resolve a tensor name to its shard file path.
+    public func shardPath(for tensor: String) -> String? {
+        guard let shard = weightMap[tensor] else { return nil }
+        return (baseDir as NSString).appendingPathComponent(shard)
+    }
+
+    /// List all tensor names.
+    public var allTensors: [String] { Array(weightMap.keys) }
+}