Stage 1.17: cellpycore.curves — spec'd capacity/OCV curve extraction

[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

Gated on the #438 decision (curve-schema home): a curves.py module + CurveCols
schema + docs/data_format_specifications/curve_table.md, porting get_cap /
get_ccap / get_dcap / get_ocv selection logic (interpolation, taper trimming,
forth-and-forth, steptable override) polars-native and schema-injected — the
extractors.py introduction pattern (issue #23) applied to curves.

Why

The most-used data API in the cellpy ecosystem currently returns frames with unspec'd,
hard-coded column names (headers report §5: "an implicit contract between cellreader
and four utils modules"). Moving it into core with a spec closes gap G4 and unblocks
utils waves 2–3. The Stage-0 snapshots (jepegit/cellpy#433) are the parity oracle.

Links (workspace)

• architecture-plan/cellpy2-loader-port-and-extraction-plan.md (§2.3, §5, §6 curve-drift risk)
• architecture-plan/hardcoded-column-headers-report.md (§5)
• Depends on: Stage 0.11: Decision register — the calls that gate Stage 1+ jepegit/cellpy#438 decision 2; oracle from Stage 0.6: Curve-extraction golden snapshots (get_cap family) jepegit/cellpy#433.

Acceptance

• Spec doc + CurveCols + spec-conformance test (the test_config_columns.py pattern).
• Parity vs the #433 snapshots across the option matrix; intentional differences
  documented, not silent.
• Release tagged for cellpy re-pin.

[jepegit]

Unblocked (2026-07-10): jepegit/cellpy#438 decision 2 confirmed the proposal — cellpycore.curves + spec-first CurveCols (docs/data_format_specifications/curve_table.md), recorded in the loader/extraction plan §2.3 (merged to cellpy/architecture-plan main). The parity oracle (jepegit/cellpy#433 curve snapshots) is committed on the cellpy side. This issue is ready to plan.