Add WaterSIC KV-cache quantization plot example

Self-contained script that reproduces the KV-cache quantization plots
comparing RTN, GPTQ, and WaterSIC methods across temperature, power-law,
retrieval, sink, and conditioning sweeps. Imports WaterSIC from the
ported zsic module and inlines baseline quantization utilities.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Kai Xu <kaix@nvidia.com>

Author: kaix-nv