Re-applied f32 IS-weights permanently. The f32_to_bf16_cast kernel had a
manual RNE rounding overflow bug producing NaN for edge-case values.
Fixed cast to use __float2bfloat16 intrinsic, but NaN persists from
a DIFFERENT source: done=nan in replay buffer (not IS-weight).
Debug printf in MSE loss kernel shows:
- done=nan (bf16 replay buffer corruption)
- is_weight negative (should be [0,1] — reduce_max atomicMax bug with negative floats)
- avg_mse=nan cascading from done=nan through Bellman target
Root cause: replay buffer done flag corruption — likely a buffer overwrite
from an adjacent allocation. Need compute-sanitizer to find the OOB write.
895/895 unit + 359/359 ml-dqn tests pass.
Smoke tests: intermittent (NaN from corrupted done flags in replay buffer).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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