Two silent bugs, both found by diffing against upstream running on CPU torch. The flow
models have no sparse conv, so upstream is runnable here with flash-attn swapped for
SDPA - a real numerical oracle, unlike the sparse path.
1. rope_phases ships as COMPLEX64 (torch.polar), so the rotation is a complex multiply
and cos/sin are the phase's real/imag parts. Taking cos() of a complex phase was
completely wrong. Verified against torch's view_as_complex formulation to 1.2e-7.
2. Upstream applies qk RMS norm BEFORE RoPE; I had it reversed. They do not commute -
RMS applies a per-component gain, RoPE rotates within pairs. Reversed, one block
still correlated 0.9998, which compounded to 0.84 across 30 blocks.
After both: SparseStructureFlowModel matches upstream at correlation 1.00000000,
max abs diff 1.2e-5, on the real 1.3B checkpoint.
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