H1: cellpy-core doc-sync pass (F7)

[jepegit]

Epic H — Housekeeping · cellpy-core doc-sync pass (F7). Gap analysis F7.

~1 hour. Fix stale "partly done" statuses in the cellpy-core guiding docs (integration roadmap STEP-12/#54, column-headers-review.md header_mapping path, make_step_table "NOT ported" line). Stale statuses cost every future planner a re-derivation.

Part of #696.

[jepegit]

Done via cellpy/cellpy-core#144 (merged).

The F7 doc-sync had three stale statuses; on review two were already resolved by the earlier cellpy-core doc-sync (#114), leaving one genuinely-stale item:

• STEP-12 (unit-handling boundary) — was 🟡 partly done, blocked on "cellpy still keeps duplicate converters (delegation pending)". That cellpy-side delegation has since landed (Stage 1.6: Units Phase 2 — delegate the duplicated converters to cellpycore.units #451 — get_converter_to_specific / nominal_capacity_as_absolute / convert_value / calculate_scaler now delegate to cellpycore.units). cellpy-core#144 flips STEP-12 to ✅ across the roadmap (summary table, section status, STEP-13+ preamble, mermaid graph). STEP-06 (golden fixtures) remains the only intentionally-continuous partly item.
• column-headers-review.md header-map path — already correct (src/cellpycore/legacy/mapping.py), fixed by Update cryptography to 2.3 #114; re-verified against current main.
• make_step_table "NOT ported" finding — already retired by Update cryptography to 2.3 #114 (now "ported and delegated, STEP-08 ✅"); re-verified.

Closes the last open item under #696.