Vector index storage and loading for frankensearch.

This crate implements the FSVI binary format reader/writer plus exact brute-force top-k vector search, with optional HNSW ANN acceleration.

FSVI File Layout

All multi-byte integers are little-endian. The vector slab is 64-byte aligned for cache-line / SIMD friendliness.

┌───────────────────────────────────────────┐
│ Header (variable length)                  │
│   magic: b"FSVI"              (4 bytes)   │
│   version: u16                (2 bytes)   │
│   embedder_id_len: u16        (2 bytes)   │
│   embedder_id: [u8]           (variable)  │
│   embedder_revision_len: u16  (2 bytes)   │
│   embedder_revision: [u8]     (variable)  │
│   dimension: u32              (4 bytes)   │
│   quantization: u8            (1 byte)    │
│   reserved: [u8; 3]           (3 bytes)   │
│   record_count: u64           (8 bytes)   │
│   vectors_offset: u64         (8 bytes)   │
│   header_crc32: u32           (4 bytes)   │
├───────────────────────────────────────────┤
│ Record Table                              │
│   record_count × 16 bytes each:           │
│     doc_id_hash: u64          (8 bytes)   │
│     doc_id_offset: u32        (4 bytes)   │
│     doc_id_len: u16           (2 bytes)   │
│     flags: u16                (2 bytes)   │
├───────────────────────────────────────────┤
│ String Table                              │
│   Concatenated UTF-8 doc_id strings       │
├───────────────────────────────────────────┤
│ Padding (to 64-byte alignment)            │
├───────────────────────────────────────────┤
│ Vector Slab                               │
│   record_count × dimension × elem_size    │
│   (2 bytes/elem for f16, 4 for f32)       │
└───────────────────────────────────────────┘