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feat(transpiler): fuse `InitializeModes` + `OrbitalRotation` into `PrepareSlaterDeterminant` (#205)

* feat(transpiler): fuse InitializeModes+OrbitalRotation into PrepareSlaterDeterminant

An InitializeModes immediately followed by an OrbitalRotation is a Slater
determinant preparation. Recognizing that pattern lets a later synthesis stage
lower it with the reduced rectangular Givens decomposition rather than the full
square orbital rotation -- but only if the two gates are first fused into a
single PrepareSlaterDeterminant, since the plugin dispatches on gate type.

This pass does that fusion. It stays deliberately conservative and
adjacency-based so the rewrite is easy to verify correct: a rotation fuses only
when the InitializeModes is its sole DAG predecessor across all its wires (so
nothing intervenes), and only the three spin-sector shapes from the plan are
matched -- full-register/spinless, per-sector, and a global init split by two
per-spin rotations. Because PrepareSlaterDeterminant is validate-then-rotate
under simulation, the rewrite is a no-op on the simulated state and only unlocks
the cheaper synthesis; anything not matching is copied through untouched (in
particular a half-covered init is left alone rather than dropping its validation
on the un-rotated modes).

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

* feat(transpiler): run the Slater-prep merge in the preset JW pipeline

Wire MergeSlaterDeterminantPreparation into the preset Jordan-Wigner pass
manager's optimization stage so an InitializeModes followed by an
OrbitalRotation is fused and lowered with the reduced Slater decomposition by
default -- the user no longer has to build a PrepareSlaterDeterminant by hand or
assemble the pass manager themselves to get the cheaper synthesis.

This is safe to enable by default precisely because the merge is a no-op on the
simulated state (PrepareSlaterDeterminant is validate-then-rotate): it only ever
changes how the gate is synthe... (continued)

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