Stage 1.5: Units Phase 1 — one CellpyUnits, one pint registry, rename the `core` alias

[jepegit]

Part of Stage 1 — behavior-preserving construction (see the Stage-1 tracking issue). Stage 0: #439. Plan documents live in the shared workspace: cellpy-workspace/architecture-plan/ (the architecture-plan repo; formerly architecture-plan/) (alongside the cellpy and cellpy-core repos). Everything in Stage 1 lands on master with the full suite green; nothing flips user-visible behavior.

Goal

• cellpy.parameters.internal_settings.CellpyUnits becomes a re-export of
  cellpycore.units.CellpyUnits (deprecated alias one release).
• Delete _ureg/ureg/Q/get_ureg from cellpy/readers/data_structures.py;
  re-export cellpycore.units.Q. Fix the direct importers
  (instruments/neware_xlsx.py:18, utils/plotutils.py:5045).
• Rename the from cellpy.readers import data_structures as core alias in
  cellreader.py:33 (it is guaranteed to confuse now that cellpycore exists).

Why

cellpy currently runs two pint registries in one process (cellpy's own +
cellpy-core's memoized one); quantities from different registries cannot interoperate —
a latent trap the Stage-0 interop test (#431) documents as a strict xfail. This issue
flips that test to a hard pass. It also makes the #378 contract test a trivial identity.
Sequencing: land before Stage 1.2/1.3 — this PR touches many cellreader lines and
must not interleave with the code moves.

Links

• architecture-plan/unit-handling-cellpy2-plan.md (Phase 1, §7 risk 1, §8 cross-check 6)
• Depends on: Stage 0.4: Unit-handling test groundwork — registry interop, converter parity, pint-optional guard #431 (the xfail exists); blocks: Stage 1.2 by agreed order.

Acceptance

• Registry-interop test passes (xfail marker removed).
• One pint.UnitRegistry per process (assert via cellpycore.units.converters._get_unit_registry).
• Full suite green; no numeric changes anywhere.