Physical fused-contraction 'einsum' operator (DataFusion ExecutionPlan)

[alxmrs]

The one open piece of the performance story (still to be spiked). An aggregate-HashJoin that computes a grouped contraction Σⱼ A·B WITHOUT materializing the full join, dispatching to a matmul/BLAS kernel on the dense path — implemented as a DataFusion ExecutionPlan. Keeps the model pure-logical (one value per cell — the portable surface) while pushing BLAS-class speed into the physical plan; deliberately does NOT adopt Tang et al.'s chunked-tensor values. (design.md §4.1)

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_{Curated from the project roadmap; see docs/design.md §8 for the milestone plan.}