Diagonal QN operator selection

In TRDH for example, we should use a symbol to indicate which diagonalQN operator to choose, instead of the multiple booleans.

Start by locating TRDH and its diagonalQN operator-selection code; inspect how the current booleans are passed and how the diagonalQN choice is represented. Done means the selection uses a symbol rather than multiple booleans, with existing behavior preserved.

Written by the indexing model from the issue text.