- New tests/test_polar_quant.py: 25 tests covering:
* Encode/decode roundtrip (cosine similarity across d=128/256/512)
* Self-inner-product preservation (auto-correlation)
* Walsh-Hadamard transform orthogonality and norm preservation
* Codebook correctness (16 centroids, monotonic, centered)
* Bit packing: 2×4-bit indices per byte
* Edge cases: zero, constant, alternating-sign vectors
* Compression ratio: 4 bits/dimension
Implementation: pure-Python reference (no numpy required for most tests,
but numpy used for vector math convenience). All thresholds calibrated
against C++ llama-turbo.cpp baseline (roundtrip_test.cpp).
Closes#54
