Add train_gpt_s9_caseops_lqer.py — S9 + CaseOps + LQER variant

S9 stack extended with caseops dataset support and LQER (Low-rank
Quantization Error Rescue). 4487 lines vs train_gpt_s9.py's 4363.
This is the script used in PR openai#1851 stage 1/2 ablations (cells A0–F4
in stage 1, Z0/P*/Q*/R* in stage 2).

Author: lijuncheng16