fix: CausalLMOutputWithPast return + audit verification

fix(v12): resolve 6 defects from v11 audit (F-NEW-6, S-NEW-5, M-NEW-5/6, MI-NEW-4/5/6)

fix(v11): resolve remaining 5 defects (S-NEW-1/2, M-NEW-2/3, MI-NEW-1)

fix(v10): resolve 5 fatal + 2 serious + 2 moderate + 2 minor defects

fix: v6 audit - all critical/severe/moderate/minor defects

fix: F1 mask double-conversion NaN, F2 think token old format, S3 vocab sync, S1/S2 TODO, N1-N5

fix: v5 remove empty tokenizer.json, unify think tokens, integrate tokenizer into training pipeline

feat: train SentencePiece tokenizer + update README

chore: replace author real name with GitHub username

fix: M2/S2/S4/M4 defect repairs

fix: v3 comprehensive defect repair