Abhayn01/arka-stage1-universal-assistant-v5

Fix EOS/repetition identity overfit | identity=73.3% | general=54.5% | nonrepeat=58.2%

Strong ARKA identity lock | identity=100.0% | retention=90.0%

Add ARKA identity to Stage1-V5 with conservative last-2-block SFT

ARKA Stage-1 universal assistant SFT v5

initial commit