Avoid copying DWG object memory stream

[Michaelliv]

This avoids repeated full-buffer copies while reading DWG objects.

DwgObjectReader stores _memoryStream as a Uint8Array, but several hot-path calls passed new Uint8Array(this._memoryStream) to DwgStreamReaderBase.getStreamHandler(...). That creates a copy of the full object stream every time object metadata is read.

On a 4.6MB AC1032 DWG, this caused DwgReader.readFromStream() to time out after 180s locally with heavy allocation/GC. Passing the existing _memoryStream view directly reduced the same read to ~2.6s.

Validation:

npm run build
npm test

Fixes #2.