Summary

Add a JSON metadata section to the APR binary format, similar to SafeTensors.

Motivation

• Enable single-file WASM deployment with embedded vocab, config, tokenizer
• Generalizable to any model type (Whisper, LLMs, etc.)
• Maintains APR benefits (LZ4 compression, streaming) while adding flexibility

Format Change

[MAGIC][HEADER][METADATA_LEN (u32)][JSON_METADATA][TENSOR_INDEX][TENSOR_DATA][CRC]

Acceptance Criteria

• Add metadata_len field to header
• Serialize/deserialize JSON metadata
• Reader/Writer APIs for metadata
• 95% test coverage maintained
• Update book documentation
• Add example: cargo run --example apr_with_metadata

Related

• whisper.apr needs vocab embedding for WASM demos
• Aligns with SafeTensors ecosystem patterns