Fuse the pusher preamble into one CUDA kernel

[max-models]

Stack: part 2 of 14, based on #709 (merge that first), next: #711. Full order: #709 → #708 → #711 → #712 → #705 → #713 → #714 → #718 → #720 → #721 → #724 → #725 → #723 → #722.

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Fuse the pusher preamble into one CUDA kernel

On the CuPy backend, the three column-slice assignments at the start of Pusher._push

markers[:, first_pusher_idx:first_shift_idx] = markers[:, : 3 + vdim]
markers[:, first_shift_idx:residual_idx] = 0.0
markers[:, residual_idx:-2] = 0.0

each sweep the whole row-major marker buffer, because the slices are strided. With 4M markers (an 8M-row buffer of 1.66 GB), this took 4.6 ms per pusher call, more than the push kernels themselves.

Changes

• New src/struphy/pic/pushing/prepare_push_cuda.cu: one thread per entry of the contiguous column range [first_pusher_idx, n_cols - 2), so all three assignments are done in a single coalesced pass.
• Pusher._push calls it on the CuPy backend; the kernel is compiled once, on first use. The NumPy path is unchanged.

This roughly speeds up the preamble by 2x.

The next big improvement for the markers would be #707