Build the read-only per-source preview graph the FE drawer's Preview tab reads (decoded from the same JSONL the commit path uses)

{
  "session_id": "string",
  "status": "string",
  "summary": {
    "row_count": 0,
    "asset_count": 1,
    "investor_count": 1,
    "new_assets": 1,
    "new_investors": 1,
    "error_count": 0,
    "warning_count": 0,
    "date_range": {
      "from": null,
      "to": null
    }
  },
  "tx_types": [
    {
      "key": "string",
      "row_count": "string",
      "color": "string",
      "error_count": "string",
      "sub_types": [
        {
          "key": "string",
          "row_count": "string"
        }
      ]
    }
  ],
  "assets": [
    {
      "id": "string",
      "label": "string",
      "exists_in_coreos": true,
      "coreos_asset_id": 1,
      "row_count": "string",
      "error_count": "string",
      "source_sessions": [
        null
      ]
    }
  ],
  "investors": [
    {
      "id": "string",
      "label": "string",
      "exists_in_coreos": true,
      "coreos_investor_id": 1,
      "row_count": "string",
      "error_count": "string",
      "source_sessions": [
        null
      ]
    }
  ],
  "rows": [
    "string"
  ],
  "sources": [
    null
  ]
}