feat: add flash pipeline kernel cases

kernels/wu_arch_cases/case20_flash_bwd_fused/README.md

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+# case20_flash_bwd_fused
+
+FlashAttention backward-style fused pipeline smoke test.
+
+The tensor warp performs one score MMA, then waits for the scalar warp to run
+softmax plus dsoftmax on the TMEM C row.  The scalar warp writes the dS row back
+to TMEM A using signed fp16 values.  The tensor warp then performs four more
+MMA steps, for five MMA operations total in this case.
+
+This case verifies:
+
+- tensor warp MMA sequencing around a scalar TMEM handoff;
+- scalar-only `FEXP.S` use for stable softmax;
+- dsoftmax shape `dS = P * (dP - sum(P * dP))`;
+- signed scalar TMEM stores feeding later tensor MMA operations.
+
+The input score row is uniform, so `P = 1/32`.  The synthetic upstream gradient
+uses two buckets, producing exact dS values `-1/32` for row entries 0..15 and
+`+1/32` for row entries 16..31.