feat(bgz17): RaBitQ encoding, PCDVQ weighted distance, sigma-band palette D1: rabitq_compat.rs — RaBitQEncoding struct with: - OrthogonalMatrix (Hadamard rotation) - encode(): normalize → rotate → sign-quantize → binary + corrections - distance_rabitq(): unbiased L2² estimate with correction factors - distance_palette(): O(1) matrix lookup - distance_corrected(): generative decompression with LFD correction D6: Base17::l1_weighted() — PCDVQ direction-weighted L1 distance. dim 0 (sign) weighted 20×, dims 1-6 (exponent) 3×, dims 7-16 (mantissa) 1×. DistanceMatrix::build_pcdvq_weighted() uses weighted distances. D7: Palette::from_sigma_bands() — training-free codebook from empirical distribution. Equal-percentile bands = GQ-style guaranteed uniform utilization. No empty clusters by construction. 114 tests pass (was 108), 6 new tests for the added features. https://claude.ai/code/session_01CdqyUTUfjKZuk8YGJzv6LB#33