Explicitly conserve symmetries

[AlexSchuckert]

Build in symmetries into the construction of the pauli words so that truncation does not violate them.

1. U(1) symmetry (total z magnetization): not done yet - would be new! Very useful for e.g. XY or Heisenberg Hamiltonian time evolution.
2. Translational symmetry: https://arxiv.org/abs/2512.12094

Codex plan:
pauli_propagation_sector_trace.pdf