Fix FrozenInstanceError when discarding non-reactant monomers

[Copilot]

MonomerEntry is @dataclass(frozen=True), but non_reactant_selection was directly assigning monomer.status = 'discarded', which raises FrozenInstanceError at runtime.

Changes

• non_monomer_detector.py: Replace all three .status mutation sites with dataclasses.replace(), rebuilding simulation_setup.monomers via list comprehension with ID-based set lookups

# Before — raises FrozenInstanceError
for monomer in non_reactants_list:
    monomer.status = 'discarded'

# After — creates new instances, updates monomers list in one pass
discard_ids = {m.id for m in non_reactants_list}
simulation_setup.monomers = [
    replace(m, status='discarded') if m.id in discard_ids else m
    for m in simulation_setup.monomers
]

• Updated the docstring note that previously claimed in-place mutation to reflect the new immutable replacement pattern.

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