Qwen3-Next: sequential GDN paths (lax.scan recurrence, solve_triangular) bound TPU step time

[bzantium]

Problem

In the Qwen3-Next Gated Delta Net (jax_chunk_gated_delta_rule), the two sequential computations dominate TPU step time:

• the inter-chunk recurrence runs as lax.scan, so the recurrent state round-trips HBM on every chunk step, and
• the UT-transform inverse A = (I+S)^-1 uses solve_triangular, which substitutes row by row and barely uses the MXU.

Neither overlaps with other work, so they bound the step directly.

Measurement

v5e-256, Qwen3-Next dense-8B config, seq 4096, bs 1, remat full (details in #4348):

build	step time	tokens/s/device
main	3.373s	1,214
#4348 (Pallas kernels)	2.571s	1,593 (+31%)

Loss trajectories match to bf16 rounding level.