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perf(transpiler): peel disjoint unit-orbitals in Slater synthesis (#203)

The reduced Givens sweep in GivensDecompositionSlaterDeterminantSynthesis
seeds a leading-m reference and transports occupation onto the physical
target modes. When an occupied orbital is a basis vector on a mode disjoint
from every other occupied orbital, that transport is realized as a chain of
bare SWAPs (XXPlusYYGate with c=0) -- pure classical relabeling that Qiskit's
generic transpiler cannot remove (a SWAP is a nontrivial unitary and the JW
preset carries no coupling map), so it must be avoided at synthesis time.

Peel off each such disjoint unit-orbital: place its X gate directly on the
target mode and exclude it from the decomposition, handing only the
genuinely-mixing remainder to givens_decomposition_slater. The detection is
basis-independent -- mode k peels iff column k of the occupied-space
projector P = A^dagger A equals e_k -- and generalizes the special case where
the whole occupied space is a signed permutation (then every orbital peels
and no XXPlusYYGate is emitted at all).

The peel helper returns an explicit (peel_modes, kept_modes, reduced) triple
so a later reference-reseeding optimization -- which would also minimize the
transport SWAPs within the mixing remainder -- can build on it without rework.

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

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