Stage 1.14: legacy mapping extensions — postfix expansion + attribute-level table

[jepegit]

Part of Stage 1 — behavior-preserving construction (see the Stage-1 tracking issue). Stage 0: jepegit/cellpy#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

Two additive pieces in cellpycore.legacy.mapping:

• expand_specific_columns(rename, modes) — the {col}_{gravimetric|areal|absolute}
  postfix expansion currently inlined in OldCellpyCellCore.add_scaled_summary_columns
  (cell_core.py:1086–1092), lifted so the bridge and the future file-importer share it.
• LEGACY_ATTR_TO_SCHEMA — attribute-level table (legacy attribute name → native
  schema path, e.g. voltage_txt → raw.potential), needed because the value-based
  mapping cannot serve the accessor shim; ~60 attributes cover everything the utils use.
  Handle the duplicate-value pair (HeadersSummary.discharge_capacity vs
  discharge_capacity_raw) explicitly.

Why

cellpy's translate.py (Stage 1.15) and later the deprecation shim consume these; the
mapping module stays the single source of truth (no header knowledge re-appears in the
bridge or the importer). Same totality-test discipline as the existing tables: an
uncategorized attribute fails the build.

Links (workspace)

• architecture-plan/cellpy2-native-headers-migration-plan.md (Phase 1, D6)
• architecture-plan/legacy-cellpy-core-header-swapping.md (mapping guarantees)

Acceptance

• Totality + bijectivity tests extended; bridge re-pointed at expand_specific_columns
  with byte-identical goldens; release tagged for cellpy re-pin.