[PB-5942]: feat/better-concurrency-handling

[jzunigax2]

Implements fs.OpenChunkWriter so rclone controls upload concurrency and buffering,
fixing OOM kills on large file uploads (rclone#9162).

• Files >= chunk_size use multipart upload via lib/multipart.UploadMultipart()

• Smaller files continue using single-part UploadFileStreamAuto()

• All encryption stays in the SDK — zero crypto/* imports in the backend

  New options

  Option	Default	Description
  --internxt-chunk-size	30Mi	Chunk size for multipart uploads
  --internxt-upload-concurrency	4	Concurrent chunks per file

[giovariot]

Niiiiice :) when could we expect this to be integrated into the official rclone? Thanks a lot

[jzunigax2]

hey @giovariot, I'd hope to get this in for rclone's next 1.74 release. I'll be submitting a PR there after this goes through some internal QA