feat(simd): re-export f32_to_bf16_batch_rne / f32_to_bf16_scalar_rne Makes the pure AVX-512-F RNE routines from commit c489d31 reachable as ndarray::simd::f32_to_bf16_batch_rne and ndarray::simd::f32_to_bf16_scalar_rne for consumer code in lance-graph. Without this re-export, callers would have to reach into the private simd_avx512 module path, which is not pub mod in lib.rs. Doc comment on the re-export explicitly pins the workspace-wide "never scalar ever" rule for F32→BF16: consumer hot loops use f32_to_bf16_batch_rne exclusively (500-20,000× faster than scalar via AMX/AVX-512-BF16 tiles), and f32_to_bf16_scalar_rne is exposed only as a unit-test reference implementation. Cross-references the Certification Process section in lance-graph/CLAUDE.md. Companion commit in lance-graph updates seven_lane_encoder.rs Lane 6 to call the batch primitive instead of its previous element-wise truncation loop. https://claude.ai/code/session_019RzHP8tpJu55ESTxhfUy1A#88