fix: bound IVF-RQ serialization allocations

[jackylee-ch]

Summary

write_ivfrq_index materialized a full byte copy of each f32 section before writing it. The quantizer centroid section is nlist * d floats, bounded only by MAX_SECTION_ELEMENTS, so serializing a large index could allocate several GiB transiently on top of the resident centroids. Stream those sections through one reusable 64 MiB write buffer, mirroring #77 for IVF-Flat.

Testing

• New ivfrq_chunked_writer_preserves_format_and_bounds asserts byte-identical output and a bounded maximum write. Without the buffer the centroid section goes out as one 32768-byte write and the test fails.
• cargo test --release --workspace (503 passed), including storage_format_fixtures
• cargo fmt --all -- --check, cargo +1.98.0 clippy --release --all-targets --workspace -- -D warnings

Notes

No public API, index option, or file-format change; output is byte-for-byte identical and independent of chunk boundaries. The per-list code and factor buffers built by block_list are unchanged.