mamba2_scan_backward kernel reads d_h_enriched [B, SH2] but was passed
self.grad_buf [TOTAL_PARAMS] — wrong buffer, wrong size. At batch_size=4096
the kernel read 1M floats from a 582K buffer → 2749 OOB reads.
Fixed: use self.bw_d_h_s2 [B, SH2] which is the actual trunk activation
gradient from the cuBLAS backward pass.
Also increased smoke test batch_size to 4096 to catch scale-dependent OOB.
compute-sanitizer: 0 errors at batch_size=4096.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
10d88e1b2a