legacy mapping: no entries for the energy columns (charge_energy_txt -> cumulative_charge_energy)

[jepegit]

Summary

cellpycore.legacy.mapping.LEGACY_ATTR_TO_SCHEMA["raw"] has no entries for the
energy columns, although both sides exist:

• native schema defines cumulative_charge_energy and
  cumulative_discharge_energy;
• cellpy's loader configurations declare charge_energy_txt and
  discharge_energy_txt, and vendor files carry the data (Maccor Watt-hr,
  Neware energy columns, …).

>>> from cellpycore.legacy import mapping
>>> {k: v for k, v in mapping.LEGACY_ATTR_TO_SCHEMA["raw"].items() if "energy" in k}
{}

Why it matters now

Found while porting loaders to the harmonize() framework
(jepegit/cellpy#559 → #560). Consequences today:

1. On the native runtime, to_native() passes the unmapped columns through
   under their legacy names, so a native frame carries charge_energy
   while the schema says the column should be cumulative_charge_energy.
   Asking c.schema.raw for energy finds nothing.
2. harmonize() drops undeclared columns by design, so a ported loader would
   silently lose energy data unless it declares a passthrough — which is the
   wrong long-term answer for a column the schema already has a name for.

Ask

Add the mapping entries:

legacy attribute	native column
charge_energy_txt	cumulative_charge_energy
discharge_energy_txt	cumulative_discharge_energy

Please confirm the reset-granularity convention for the energy columns matches
the capacity one (cycle-cumulative per direction, per the harmonized-raw spec's
Capacity convention) — the cellpy side will normalise them the same way.

Also unmapped (for a decision, not necessarily this issue)

These native raw columns have no legacy attribute mapped to them. Several are
correctly framework-owned or native-only, but datetime_txt → epoch_time_utc
looks like a genuine omission of the same kind:

epoch_time_utc, step_type, step_mode, step_type_detail, cycle_type,
ref_potential, step_charge_power, step_discharge_power, aux_*,
source_*, mask, test_id.

Cross-repo order per the cellpy 2 release plan §3: core PR → core release →
cellpy re-pin → cellpy consumes.