v2: enable renumber_cycles=False (original per-test cycle numbers) in campaign merge

[jepegit]

Context

CellpyCell.merge(..., mode="campaign", renumber_cycles=False) raises
NotImplementedError with the message that it "requires per-test summary
support through the core bridge; tracked for the native-schema path (#511)"
(cellreader.py, campaign branch of merge()).

That blocker is gone. cellpycore 0.2.2 (core #136) delivered exactly the
missing pieces on the legacy bridge:

• test_id is carried through the legacy steps and summary output,
• the legacy summary cruft (cumulated CE, shifted_*, RIC) is windowed per
  test_id,
• and on the cellpy side, PR Fix cellpycore 0.2.2 sync fallout: merger summary test_id + stale tests #524 made the campaign merger remap the
  right-hand summary's test_id and removed the v2: composite group keys + multi-file merge (V2-03/07) #507 re-stamp workaround.

So the per-test grouping/windowing machinery the flag needs now exists end to
end on the legacy path — ahead of the native-schema runtime (#511 shipped the
opt-in, but campaign merge is not supported on native-schema cells yet).

Work

• merger.campaign_fold: make the cycle offset conditional on
  renumber_cycles (data-point offsets must stay unconditional — raw
  data_point must remain globally unique for fid/step lookups).
• Drop the NotImplementedError guard in CellpyCell.merge and forward the
  flag.
• Decide/document consumer semantics for duplicate cycle numbers: with
  original numbering, (test_id, cycle) is the key — cycle-keyed consumers
  (get_cap(cycle=...), split_many, with_cycles, exporters) will see
  duplicate cycle numbers. Minimum bar: document that these operate on the
  union of matching cycles; anything smarter is follow-up.
• Tests: campaign merge with renumber_cycles=False on the two-res-file
  fixture (tests/test_merge_campaign.py) — raw/steps/summary keep original
  per-test cycle numbers, test_id values stay {0, 1}, recomputed summary
  windows per test (cumulatives reset at the test boundary), and a v9
  round-trip preserves everything.

Refs

• core Make the C-rate available in the summary file #136 (cellpy/cellpy-core) — the bridge changes; shipped in 0.2.2
• v2: composite group keys + multi-file merge (V2-03/07) #507 (campaign merge), v2: native schema opt-in path (V2-11) #511 (native schema opt-in), PR Fix cellpycore 0.2.2 sync fallout: merger summary test_id + stale tests #524
  (summary test_id remap + workaround removal)

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