Support uploading clean JSON without base64 metadata wrapping

[crtahlin]

Problem

client.upload() always wraps content in a metadata structure:

{
  "data": "<base64 of content>",
  "content_hash": "<sha256 of content>",
  "stamp_id": "..."
}

This is the SWIP-defined wrapper, but some use cases need to store clean JSON directly — e.g. dataprovenance-app stores a file-proof record (just a hash + metadata) and wants:

• The JSON readable on Swarm without base64 decoding
• The data field to contain the structured record directly (not base64)
• Compatibility with gateway notary signing (which hashes the data field)

Current workaround

dataprovenance-app bypasses client.upload() entirely and POSTs directly to the gateway via fetch, using only client.acquireStamp() from the SDK.

Proposed solution

Add a raw or passthrough mode to upload() that skips the base64/metadata wrapping:

await client.upload(proofJson, { raw: true, sign: 'notary' });
// Uploads the JSON as-is, no wrapping
// Gateway can still sign it (requires data field)

Or a dedicated method:

await client.uploadDocument(proofRecord, { sign: 'notary' });
// Wraps in { data: proofRecord } if not already wrapped
// No base64 encoding

Context

• dataprovenance-app stores file-proof records (hash only, no file content)
• Gateway notary signing requires a data field in the JSON
• The SWIP allows custom provenance standards — data can hold arbitrary structured content
• Related: add SWIP-37 Swarm-Based Data Provenance Framework ethersphere/SWIPs#71

[crtahlin]

Implemented in branch feature/raw-upload-and-storage-ref.

Changes:

• Added raw: true option to upload() — skips base64/metadata wrapping, embeds JSON directly in the data field
• Added DocumentMetadata type (data is Record<string, unknown> instead of base64 string)
• Added downloadDocument() method for retrieving raw JSON documents
• Added buildDocumentMetadata() and verifyDocumentHash() utility functions
• content_hash is SHA256 of JSON.stringify(data) for consistency and notary compatibility
• Reference app updated with raw JSON upload/download mode toggle

Usage:

// Upload raw JSON
const result = await client.upload(
  { file_hash: 'abc123', filename: 'report.pdf' },
  { raw: true, sign: 'notary' }
);

// Download raw JSON document
const doc = await client.downloadDocument(result.reference);
console.log(doc.document); // { file_hash: 'abc123', filename: 'report.pdf' }

282 unit tests passing.