Remove pub state_dim field from DQNConfig and GpuReplayBufferConfig; remove the
state_dim field from GpuExperienceCollector. Replace all reads with
ml_core::state_layout::STATE_DIM (and STATE_DIM_PADDED for cuBLAS-padded
strides). Checkpoint loading now validates saved state_dim against the
constant and hard-errors on mismatch. GpuAttentionConfig.state_dim is a
distinct attention-feature dim and is left untouched.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>