All committers have signed the CLA.

I’m going to keep this as a draft for now while I do a deeper local validation pass.

In addition to the committed tests, I want to check a few things more exhaustively before marking this ready for review:

• random spin-balanced UCJ operators against the existing UCJOpSpinBalancedJW -> PRE_INIT -> decompose path
• random state evolution checks, including fixed particle-number sectors
• restricted interaction_pairs cases for both same-spin and opposite-spin interactions
• t2-based initialization across zero, dense, sparse, padded, and multi-repetition inputs
• parameter ordering and binding behavior on the generated Qiskit circuit
• validation behavior for bad inputs and duplicate/lower-triangular interaction pairs

The main thing I want to be sure of is that the new parameterized circuit is not just structurally reasonable, but actually matches the existing ffsim/Qiskit LUCJ behavior across representative cases. Once that internal pass looks clean, I’ll move this out of draft.

I did the extra local validation I wanted to do before marking this ready.

Besides the tests in the PR, I ran randomized checks against the existing ffsim/Qiskit LUCJ path. That included full-state evolution, fixed particle-number sectors, restricted interaction pairs, and t2-based initialization cases. I also checked parameter ordering, parameter binding, and a few invalid-input paths.

Everything matched to numerical precision. The generated parameterized circuits agreed with the existing merged UCJ circuit behavior up to global phase, including the restricted-interaction cases.

Marking this ready for review now.