add_piecewise_formulation use case: snapshot-dimensioned breakpoints (PyPSA/PyPSA#1914) #953

Note: AI-assisted (written with Claude).

The add_piecewise_formulation docstring asks for use cases while the API is evolving, so a one-line cross-reference: PyPSA/PyPSA#1914 exercises snapshot-dimensioned breakpoints (a (snapshot, _breakpoint) DataArray per entity, verified working on 0.9.1 with the lp tangent method) to express time-varying piecewise marginal costs. Nothing to change here as far as I can tell; just flagging that the proposal relies on that broadcast behaviour staying supported.