Add search API compatible simprint format

[titusz]

Add support for outputting simprints in a format compatible with the search API schema.

Current behavior:

• Simprints use maintype, subtype, version fields
• Features organized in FeatureSet with Index-Format or Object-Format

Required changes:

• Add to_api_format() method to convert metadata to API-compatible structure
• Organize simprints by type key (e.g., SEMANTIC_TEXT_V0) instead of using separate maintype/subtype/version fields
• Keep all existing fields including optional content, offset, size

Target API Schema:

{
  "simprints": {
    "SEMANTIC_TEXT_V0": [
      {
        "simprint": "XZjeSfdyVi0",
        "offset": 0,
        "size": 512,
        "content": "optional text chunk"
      }
    ]
  }
}

Implementation Notes:

• Add to_api_format() method similar to existing to_index_format() and to_object_format()
• Consider adding CLI flag (e.g., --api-format) to output in this format
• Type key format: {MAINTYPE}_{SUBTYPE}_V{version} (e.g., SEMANTIC_TEXT_V0)