Battery carbon footprint: no methodology to implement

[LKSNDRTMLKV]

co2e::cfb::calculate_cfb returns CalcError::NotImplemented, and should keep
doing so. This records why, so the next person does not mistake it for an
oversight.

Three independent blockers, any one of which is sufficient:

1. The Art. 7(1) methodology delegated act does not exist. It was due
   February 2025 and has slipped repeatedly. Without it there is no methodology
   to implement — the expected shape is the PEF Category Rules for rechargeable
   batteries, but "expected" is not a basis for a compliance figure.

   Do not mistake Commission Delegated Regulation (EU) 2025/606 for it. That one
   covers recycling efficiency and material recovery rates.

2. carbonFootprintClass is barred from a battery passport today. The
   Commission's own data-point guidance marks it, and dueDiligenceUrl, "not to
   be filled/displayed" for every category it covers, because the implementing
   act that will specify their format has not been adopted. A value filed now
   cannot be the one that act will ask for. The rules crate encodes this and the
   sector plugin now reports it.

3. The open calculator multiplies numbers the operator chose.
   co2e::calculate is Σ(mass × EF) + kWh × gridEF over caller-supplied
   emission factors. Running it and stamping a receipt would produce an audit
   artefact attesting that we multiplied correctly — which is true, and is not
   what a carbon-footprint declaration claims.

What unblocks it

Blocker 1 lifting is necessary and not sufficient: a real CFB also needs licensed
lifecycle-inventory data, which is why FactorProvider exists as a runtime
injection point and why no dataset is bundled in this crate.

Until then the honest answer for a battery's carbon footprint is the
manufacturer's declaration, carried verbatim, which is what the passthrough
strategy does.