Add ImageMetadataBuffer helper for partial‐file XMP buffering

[klaari]

This PR introduces a new ImageMetadataBuffer helper that reads only the minimal bytes
required from a PNG/JPEG/WebP to locate XMP metadata, rather than downloading the entire file.
That reduces memory usage and bandwidth when extracting XMP from large images.

Usage example:

use CSD\Image\Buffer\ImageMetadataBuffer;
use CSD\Image\Image;

$url = 'https://example.com/photo.png';
$string = ImageMetadataBuffer::bufferUpThroughXmp($url);
$image  = Image::fromString($string);
$xmp    = $image->getXmp();