Fix missing realization curves for delta ensemble _DIFF vectors

[Copilot]

Delta ensemble plots showed only Mean statistics for _DIFF vectors; individual realization curves were missing.

Changes

RimDeltaSummaryEnsemble::ensembleSummaryAddresses()

• Generate _DIFF suffixed addresses for all vectors from both source ensembles
• Enables UI discovery of differential vectors for both statistics and realization curves

RimDeltaSummaryCase data retrieval

• Strip _DIFF suffix before cache/reader lookups via getBaseAddress() helper
• Modified timeSteps() and values() to normalize addresses:

  // Accept "FOPT_DIFF" from UI, query cache with "FOPT"
  auto baseAddress = getBaseAddress(resultAddress);
  if (needsCalculation(baseAddress)) {
      calculate(baseAddress);
  }
  return m_dataCache.at(baseAddress);

The _DIFF suffix is a presentation convention—delta cases always compute differences. This change aligns address representation between UI and internal storage.

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