simd: masked_strided_group_sum — the strided sub-word gather consumers lacked

[AdaWorldAPI]

Closes a named gap rather than adding surface speculatively.

lance-graph-java's mask-native sweep needed "gather a sub-word group out of a strided register under a runtime grouping and widen-accumulate". masked_sum_i32 is contiguous i32; eq_u32_strided_to_mask reads one aligned u32 per record. Neither covers it, so the consumer carried its own scalar loop and documented the absence as a W1a gap — added THERE, consumed here — which is the shape the "all SIMD from ndarray::simd" invariant asks for. That consumer's loop is now one delegating call.

The kernel is scalar, and the doc gives the reasoning rather than an apology

One small register per record at a large stride (512 bytes in the motivating case) is memory-bound; records are not adjacent so several cannot be vector-loaded; and widening six u16s inside one record would optimise the part that is already free. A contiguous or small-stride variant would genuinely vectorise and is named as a different primitive — not a flag on this one.

Correctness details that are easy to get wrong

• Overflow is reported, not wrapped: accumulates in i128, range-checks once. With group_bytes = 4 one record contributes up to groups × (2³² − 1).
• Sub-word loads are byte-wise: a group's offset is not guaranteed aligned for a 3-byte grouping, and an unaligned wide read is UB in Rust even where the hardware tolerates it.
• Dirty mask tails are clamped, same guard and same reason as masked_sum_i32.

Tests

Six, each falsifiable — notably that the three groupings of one register must give three different answers, so a kernel ignoring its grouping fails; plus the two contract violations (group_bytes outside 1..=4, a record reading past the buffer) as should_panic.

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