Traditional replay
------------------
turn 1: system + user 1
turn 2: system + user 1 + assistant 1 + user 2
turn 3: system + user 1 + assistant 1 + user 2 + assistant 2 + user 3

Stateful inference
------------------
wake:   prefill stable system and continuity context once
turn 1: append user 1; retain generated sequence and KV
turn 2: append user 2; retain generated sequence and KV
turn 3: append user 3; rebuild only when the causal prefix is no longer valid