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Fix SamplerQNN._postprocess to handle transpiled circuit layouts (#1041)

* fix: marginalise ancilla qubits in SamplerQNN._postprocess

  The previous filter (ki < 2**num_virtual_qubits) discarded valid
  measurements when logical qubits were mapped to high physical
  positions by the transpiler. This fix uses the circuit layout to
  extract only the logical qubit bits and marginalises over ancillas.

  Fixes #1040

* test: add regression test for SamplerQNN layout postprocessing

  Verifies that _postprocess correctly marginalises ancilla qubits
  when logical qubits are mapped to high physical positions by the
  transpiler.

  Related: #1040

* docs: add release note for SamplerQNN postprocess fix

  Fixes #1040

* style: apply black formatting

* fix: use SamplerV2 in test to trigger physical-space bitstring bug

* docs: clarify SamplerV2 usage in regression test docstring

* style: apply black formatting

* fix: use American English spelling in comments

* fix: add type annotation for probs_i to satisfy mypy

* fix: use American English in release note

* docs: update release note to describe both bug causes and add ki to spell dict

* docs: update test docstring to describe both bug causes and fix spelling

* refactor: rename ki to
  key_int for clarity in _postprocess

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