Recursive linear x y variable

`RecursiveLinearXYVariable` ¶

Bases: Variable

Recursive linearization variable for the PC-HLP model.

Reproduces the cubic term for merged (aggregated) clients:

\[X_{ijkm}^z \;\widehat{=}\; y_{ij}^z \cdot x_{ik} \cdot x_{jm}\]

\[X_{ijkm}^z \in \{0,1\} \quad \forall (i,j,z) \in K\]

Same structure as LinearXYVariable but uses aggregated client keys from BilevelDataCol.CLIENT_KEYS.