Preserve read metadata bytes on rewrite

[InternetRando1]

Fixes loss of MetaData section when roundtripping UE5.4+ assets

Steps to reproduce:

1. Open any .uasset created by UE5.4+ (ObjectVersionUE5 >= METADATA_SERIALIZATION_OFFSET)
2. Roundtrip it: new UAsset(path, engineVersion).Write(outputPath)
3. Compare file sizes — output is smaller (e.g. -53 bytes for attached test Material based on Epic tutorial M_Screen_Damage.zip)
4. Open the roundtripped file in Unreal Editor — it fails to load

Root cause:
UAsset.ReadHeader() reads MetaDataOffset (introduced in UE5.4), but the corresponding
bytes are never read or preserved. During write:

• The original MetaDataOffset is written back into the header
• No MetaData section is emitted

This causes:

• The MetaData section to be dropped
• File size to shrink
• MetaDataOffset to point to an invalid location

Observed impact:
For a UE5.7 Material asset, this removes 53 bytes of build dependency hash data.
The resulting file is rejected by Unreal Editor.

What the MetaData section contains:
Raw bytes between MetaDataOffset and ImportOffset.
In UE5.7 this includes package build dependency metadata (hash strings).
This format is currently not parsed by UAssetAPI.

Fix:
Preserve the MetaData section as raw bytes (passthrough):

• Add field: byte[] MetaDataBytes
• Read: capture bytes between MetaDataOffset and ImportOffset
• Write: emit MetaDataBytes before Imports and update MetaDataOffset

This preserves correctness without introducing format-specific parsing.
Future work could deserialize this section if needed.