# `buildGgufHeader`: GGUF header reconstruction with proper alignment

## 🎯 Why `serializeGgufMetadata`
#1740 isn't enough

**`serializeGgufMetadata`** only handles metadata serialization but
**lacks critical functionality**:

- ❌ **No tensor info preservation** - loses original tensor definitions
- ❌ **No alignment calculation** - creates invalid memory layout  
- ❌ **Incomplete headers** - generates metadata-only files
- ❌ **Framework incompatible** - resulting files can't be loaded

## 🚀 `buildGgufHeader`: Complete solution

**`buildGgufHeader`** is a **comprehensive function** that uses
`serializeGgufMetadata` internally but adds essential missing pieces:

```typescript
export async function buildGgufHeader(originalFileBlob, updatedMetadata, options) {
  // 1. ✅ Use serializeGgufMetadata for metadata
  const newHeaderBytes = serializeGgufMetadata(updatedMetadata, {
    littleEndian: options.littleEndian,
    alignment,
  });

  // 2. ✅ Preserve original tensor info (missing in serializeGgufMetadata)
  const originalTensorInfoBlob = originalFileBlob.slice(tensorInfoStart, tensorInfoEnd);

  // 3. ✅ Calculate proper alignment (missing in serializeGgufMetadata)  
  const prePadLenNew = kvEndOffset + tensorInfoSize;
  const GGML_PAD = (x, n) => (x + n - 1) & ~(n - 1);
  const targetTensorDataOffset = GGML_PAD(prePadLenNew, alignment);
  const padLen = targetTensorDataOffset - prePadLenNew;

  // 4. ✅ Reconstruct complete, valid header
  return new Blob([
    newHeaderBytes.slice(0, kvEndOffset),  // Metadata from serializeGgufMetadata
    originalTensorInfoBlob,                // Preserved tensor info
    new Uint8Array(padLen)                 // Correct alignment padding
  ]);
}
```

## 📊 Comparison

| Function | Metadata | Tensor Info | Alignment | Complete Header |
|----------|----------|-------------|-----------|-----------------|
| `serializeGgufMetadata` | ✅ | ❌ | ❌ | ❌ |
| `buildGgufHeader` | ✅ | ✅ | ✅ | ✅ |

## 🎯 Key Innovation

**`buildGgufHeader` = `serializeGgufMetadata` + tensor preservation +
alignment calculation**

This creates **production-ready GGUF files** that maintain framework
compatibility while enabling metadata updates.