v4.1.0

• Add uploads: {name => io} to Assembly options and create! for named multipart uploads, separate from signed JSON Instructions.
• Send custom fields only in signed JSON, preserving numbers, booleans, arrays, objects, and null without duplicate form fields.
• Keep IOs in legacy fields working as named uploads throughout v4, omit them from JSON, and warn to migrate to uploads before the next major release. Positional file_N uploads and create!(fields: metadata) overrides remain supported.
• Restore in-memory uploads to their original position on rate-limit retries and raise a clear IOError when a non-seekable upload cannot be retried.