simd: two-column GROUP BY — GroupKeyAddr::Pair + five _pair keyed reductions

[AdaWorldAPI]

What

Adds multi-key GROUP BY a, b to the keyed-reduction family as a third group address, not a third walker. The group of row i is the composite hi[i] * stride + lo[i], fused into group_walk, so no composite key lane is ever materialised. Every fold closure is copied unchanged from its Resident sibling.

• GroupKeyAddr::Pair { hi, lo, stride }:
  • A minor key lo >= stride names no group and is dropped. This follows the family's zero-fallback contract; it is never an error.
  • The composite is formed in u64, where (2^32-1)^2 + (2^32-2) < 2^64, so it cannot overflow. It then meets the usual k < groups drop.
• New public functions, re-exported from ndarray::simd: masked_group_{count_u32, sum_i32, sum_sym_i32, min_i32, max_i32}_pair.

This is the ndarray half of lance-graph's parity item W-D (multi-key GROUP BY). The mask-risc / quack lowering follows in lance-graph, whose CI resolves ndarray through the local path dep.

Gates (local, toolchain 1.98.1, debug=0)

• group_family_tests: 14/14 passing, 4 of them new. Doctests: 4/4. clippy --lib -D warnings clean; cargo fmt --check clean.

• Differential: all five _pair folds equal their Resident sibling fed the precomputed composite key. Fixture: n = 1000 with a dirty tail word; anti-vacuity requires ≥ 20 non-empty groups.

• Disable runs. I committed first, broke each guard, and all three went red:

  • Delete the lo >= stride guard → pair_drops_a_minor_key_at_or_past_stride fails.
  • Transpose hi/lo → three tests fail.
  • u32 wrapping arithmetic instead of u64 → the no-overflow test fails.

  The first version of the stride test was vacuous: its dropped rows also fell past out.len(), so the universe check dropped them anyway. It is rewritten so each dropped row would otherwise land in another group's slot ((0, 4) → slot 4 = (1, 0)), and it has been re-verified red → green.

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