Bump pinned cellpy-core to the polars engine and verify end-to-end parity

[jepegit]

cellpy depends on cellpycore @ git+https://github.com/cellpy/cellpy-core.git@main, and CellpyCell.make_step_table/make_summary already delegate to self.core.make_core_*. The cellpy-core engine has since been migrated to polars (cellpy-core #13, phases 1–4), but the version installed in this repo's env is stale (it predates make_core_step_table, so make_step_table currently raises AttributeError against a fresh core).

Tasks

• uv sync / force-reinstall cellpy-core from main (after cellpy-core PR Pin pyodbc to latest version 4.0.16 #20 merges) so the pinned engine includes the polars rewrite.
• Run cellpy's own test suite (conda cellpy_dev_313) and fix any integration fallout.
• Confirm cross-library parity on the canonical Arbin cell: 103 steps / 18 cycles / cyc-1 data_point 1457, and that make_summary(find_ir=True, find_end_voltage=True) produces the expected frame.
• Sanity-check make_step_table(usteps=...), bad_steps, from_data_point, and nom_cap paths still flow through the bridge.

Refs

• cellpy-core Tutorial  #13 (engine migration), PR Issue #13: re-land Phase 3b (summary engine) + Phase 4 (cross-repo step-type parity) cellpy/cellpy-core#20
• Consumer-side tracking: Prepare cellpy to consume cellpy-core (isolate the Data object via a core seam) #377 / Add header/unit parity contract tests between cellpy and cellpy-core #378