feat: safetensors header parser + streaming indexer

Parses the safetensors JSON header (no serde dependency) and produces
GgufFile-compatible types so stream_index_gguf_bf16_with_header works
unchanged on safetensors files.

Safetensors stores full BF16 weights — no quantization noise.
For the reasoning diff pipeline, BF16→Base17 gives cleaner
fingerprints than Q8_0→f32→Base17.

Includes test_stream_index_qwen35_safetensors for 11-shard
Qwen3.5-27B indexing at full BF16 precision.

Author: AdaWorldAPI