[FEATURE] Extend GPU build acceleration to the scalar-quantized Seismic indexes

Problem

GPU build acceleration only applies to SeismicIndex / DiskSeismicIndex. The scalar-quantized variants silently fall back to CPU, because both offloads are gated on float (U32) weights while quantized indexes store uint8/uint16 codes:

• nsparse/cluster/kmeans_utils.cpp:176 — assignment offload requires element_size == U32.
• nsparse/cluster/inverted_list_clusters.cpp:133 — summarize_with_gpu_<T>() bails unless T is float.

All four types share build_clustered_lists(), so nothing else blocks it. No warning is emitted, so the build looks GPU-enabled but runs on CPU.

Proposal

1. Support U8/U16 in the assignment kernel (convert codes to float on upload, or use an integer SpMM).
2. Templatize the max-pool kernel over code width (atomicMax on unsigned int replaces the __float_as_int trick).
3. Keep bit-identical CPU parity and CPU fallback; add U8/U16 parity tests.
4. Meanwhile, warn once when a GPU build sees quantized weights.

For scale: the float path gives ~6x on base_full (483s → 75s, L4 vs 32-core).

Start with nsparse/cluster/kmeans_utils.cpp:176 and nsparse/cluster/inverted_list_clusters.cpp:133, then trace the shared build_clustered_lists() path. Compare the existing float GPU path with scalar-quantized U8/U16 data and verify CPU parity, fallback behavior, and the requested one-time warning. Add parity coverage for both code widths and run the relevant build and test commands.