C.29.2:11.3 - Cost the represented objects, not the problem label

Qiskit Aer’s simulator documentation supplies a concrete current comparator: dense state-vector storage, alternative simulation representations, and controls that discard matrix-product-state coefficients. Adopt representation-sensitive counting in :4.6 and reject treating a truncation setting as an accuracy guarantee for an arbitrary observable. The :5.3 payload arithmetic and restricted product-state construction expose the relevant gain directly. The suitable general simulator remains unselected until the state family, operations, output and error are known. Reopen the calculation when those conditions or the actual algorithm change.