partial fix: Metal kernel bits=8 support - Embedding OK but Router/Expert still NaN

修复成果：
✅ 创建dequantize_row_8bit kernel
✅ Swift dequantizeRow函数检测bits并调用正确kernel
✅ loadExpertGroup groupSize计算修复
✅ Embedding测试：0 NaN/2816

待修复：
⚠️ Router/Expert forward pass仍有2 NaN
⚠️ Router matmul可能使用错误kernel
⚠️ Expert matmul可能使用错误kernel

测试对比：
修复前：Embedding 0 NaN, Forward 2 NaN
修复后：Embedding 0 NaN, Forward 2 NaN（未变）
结论：Embedding一直正常，NaN在Router/Expert/LM head

技术原理：
4-bit: packedIdx=g*(groupSize/8)+inG/8, shift=(inG%8)*4, &0xF
8-bit: packedIdx=g*(groupSize/4)+inG/4, shift=(inG%4)*8, &0xFF

下一步：
检查Router/Expert matmul是否使用8-bit kernels
或使用26B-Standard代替（完美0 NaN）

进度：60% ✅
推荐：使用26B-Standard ⭐⭐⭐⭐⭐

Sources/MarkBase/Model.swift

@@ -1591,7 +1591,9 @@ readers = readersDict
 
 func dequantizeRow(weight: QuantizedWeights, tokenId: Int,
                     output: MTLBuffer, nCols: Int? = nil) throws {
-        let pso = try engine.pipeline(named: "dequantize_row")
+        // Detect bits and use correct kernel (8-bit or 4-bit)
+        let kernelName = weight.bits == 8 ? "dequantize_row_8bit" : "dequantize_row"
+        let pso = try engine.pipeline(named: kernelName)
         
         let cmdBuf = engine.commandQueue.makeCommandBuffer()!
         let enc = cmdBuf.makeComputeCommandEncoder()!