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feat(transpiler): block-split the mixing remainder in Slater synthesis (#207)

Preparing a Slater determinant with the rectangular leading-reference Givens
sweep spends bare transport SWAPs (XXPlusYYGate with c=0) shuttling occupation
across empty modes that separate disjoint mixing clusters. Peeling (#206)
removes disjoint *unit*-orbitals but cannot touch the inter-cluster gaps of a
genuinely-mixing remainder.

Partition the post-peel mixing remainder into blocks whose physical-mode
windows [min-support, max-support] are disjoint (a classic interval merge on
the windows, mapped through kept_modes), and synthesize each block
independently on its own contiguous window via the unchanged leading
givens_decomposition_slater. The inter-cluster gaps then factor out entirely.
Grouping must be on windows, not raw support: a nested/interleaved pair
(support {0,3} and {1,2}) shares a window and must stay in one block, else an
emitted rotation crosses an occupied mode and prepares the wrong state.

Peel and block-split are complementary, not substitutes: a peeled unit is free
(a direct XGate, no rotation), so absorbing an interior unit into a block would
*increase* the two-qubit-gate count rather than reduce it. Peeling stays and
runs first, block-split handles only the mixing remainder. The #206 JW-parity
sign compensation is retained unchanged and still applies to the one
non-adjacent gate a peeled mode strictly inside a block window can force;
within a block every gate is genuinely mode-adjacent. A single mixing cluster
spanning all kept modes is one block and reproduces the previous behavior
exactly.

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

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