feat(strategy): nystrom captures 3 subspaces, not 4 (drop redundant col(B) landmark block)

[RenzoMXD]

PR kind

• fix
• feat

DRAFT -- GPU scorecard pending. No GPU available right now; opening as a draft to avoid the missing-scorecard auto-close (close_missing_scorecard fires immediately for a non-draft feat PR with no scorecard). CPU-safe validation is green. Will mark this ready for review once I attach a real python -m eval scorecard.

Summary

Mirrors #156's rsvd fix, applied to nystrom (#194).

The reconstruction Ĉ = P A P B P (P = QQᵀ) needs only col(A), row(A), row(B) in range(Q) -- col(B) is provably redundant. #156 proved this for rsvd's random-projection sketches. That proof is about the projector algebra, not about how each space gets sketched, so it applies identically to nystrom's landmark-column sketches. NystromTransform still split its landmark budget 4 ways (col(A), row(A), col(B), row(B)) -- the same pre-#156 waste rsvd had. This splits 3 ways instead, over {col(A), row(A), row(B)}.

basis_flops is unchanged (2 n m²) -- same total sketch width, just reallocated.

What's new here versus just copying #156's reasoning

Added a numeric proof specific to landmark sampling (not just assumed by analogy to rsvd's random projections): construct Q from each pair of the three spaces and confirm reconstruction breaks when col(A) or row(B) is dropped, but not when col(B) is dropped -- see test_nystrom_col_b_is_redundant_but_col_a_and_row_b_are_not.

CPU-safe validation

$ uv run python -m strategy.smoke        # 2/2 transforms pass
$ uv run --extra test python -m pytest tests/ strategy/tests/ eval/tests/ -q
270 passed, 24 skipped

GPU Result (required for feat PRs only)

Pending -- will attach before marking ready for review. Plan: same regime #156 used (--n 8192 --pairs 3 --fill lowrank --data-rank 16 --rank-m 48 --transforms nystrom,rsvd --seed 0), before/after comparison against main.

Scope

strategy/transforms.py only (Open -- "the main event" zone per CONTRIBUTING.md); no API change, basis_flops unchanged, no protected-path edits.

Checklist

• CPU-safe validation passed
• My commits do not include Co-authored-by footers for coding agents
• GPU scorecard attached (pending -- draft)