v2: Apply tokenizer UTF-8 fix + Engine writeFloats helper

- Tokenizer fix: collect <0xXX> bytes and decode as UTF-8
  (fixes Chinese/non-ASCII character decoding)
- BPETokenizer + HuggingFaceTokenizer: both updated
- Engine.swift: added writeFloats() utility method
- FloatWeights struct added to Layer.swift (bf16 support)
- attnQBits/KBits/VBits/OBits detection added to Model.swift
- bf16 layer weight support from commit 48c0347 cherry-picked

Sources/MarkBase/Layers/Layer.swift

@@ -13,6 +13,14 @@ public struct QuantizedWeights {
     public let groupSize: Int      // Quantization group size (32, 64, etc.)
 }
 
+// ── Float Weights (non-quantized bf16/f32) ────────────────────────────
+
+public struct FloatWeights {
+    public let weight: MTLBuffer   // Float32 [outDim, inDim]
+    public let inDim: Int
+    public let outDim: Int
+}
+
 // ── Layer Configuration ──────────────────────────
 
 public struct E4BLayerConfig {