Qiskit / qiskit-fermions / 30013580023

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feat(transpiler): merge runs of consecutive OrbitalRotation gates (#204)

Adds MergeOrbitalRotations, an optimization pass that collapses a maximal
run of adjacent OrbitalRotation gates on the same modes into a single gate
whose rotation_unitary is their product in circuit order. The synthesis stage
then lowers one decomposition per run instead of one per gate.

This is the qiskit-fermions analogue of ffsim's merge_orbital_rotations.
Because our OrbitalRotation is a single n x n unitary on n contiguous modes
(rather than carrying per-spin blocks in one gate), the merge is a plain
matrix product with no spin bookkeeping; only rotations on the identical mode
set merge. The pass is added to the preset Jordan-Wigner optimization stage.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

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9849 of 10082 relevant lines covered (97.69%)

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