Phase 5 ("Lethe") ships the encode-side inverse of `remember`. v0.5.0
gathers steps 1a/1b/1c (commits a5c452d / 24cba42 / ab4e735) under a
single minor-version namespace:

- `lethe` namespace introduced (encode counterpart to `remember`)
- `BnB` encode kernels: NF4 / FP4 / INT8, plain + double-quant
- Bit-exact round-trip validation harness
- Sign-of-zero preservation tweak in `dequantize_bnb4_to_bf16`
  (recovers sign info bitsandbytes Python decode discards on FP4)
- Cross-architecture validation: byte-exact round-trip on 7 fixtures
  across 4 architecture families (Llama 3.2 / Qwen3 / Qwen2.5 / Phi-3.5)
- `pub const NF4_CODEBOOK` / `FP4_CODEBOOK`
- PyTorch quantize-timing sidecars (`.timing.json`) for every fixture

README.md adds a "BitsAndBytes Quantization (Lethe -- Phase 5)" section
with the cross-architecture fixture table and the sign-of-zero finding.
CHANGELOG.md renames `[Unreleased]` to `[0.5.0] - 2026-05-17` with the
full Phase 5 entry block.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>