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/Tokenizer/BPETokenizer.swift

@@ -201,6 +201,7 @@ public final class BPETokenizer: Tokenizer, @unchecked Sendable {
     }
     
     private func decodeByteTokens(_ text: String) -> String {
+        var bytes: [UInt8] = []
         var result = ""
         var i = text.startIndex
         
@@ -215,7 +216,7 @@ public final class BPETokenizer: Tokenizer, @unchecked Sendable {
                         let hexStr = String(text[hexStart..<hexEnd])
                         
                         if let byte = UInt8(hexStr, radix: 16) {
-                            result.append(Character(UnicodeScalar(byte)))
+                            bytes.append(byte)
                             let afterHex = text.index(after: hexEnd)
                             if afterHex < text.endIndex && text[afterHex] == ">" {
                                 i = text.index(after: afterHex)
@@ -228,10 +229,18 @@ public final class BPETokenizer: Tokenizer, @unchecked Sendable {
                 }
             }
             
+            if !bytes.isEmpty {
+                result += String(bytes: bytes, encoding: .utf8) ?? ""
+                bytes.removeAll()
+            }
             result.append(text[i])
             i = text.index(after: i)
         }
         
+        if !bytes.isEmpty {
+            result += String(bytes: bytes, encoding: .utf8) ?? ""
+        }
+        
         return result
     }
 }