#14766 Fix potentially uninitialised object used for accumulation

RigStatisticsCalculator::mobileVolumeWeightedMean() accumulated into a sum
from an out-parameter that callees were not required to write. Every
implementation of the per time step overload has early-return paths that
leave the value untouched, as does RigWeightedMeanCalc::weightedMeanOverCells,
so indeterminate stack memory could be summed and presented in the UI.

Return std::optional<double> from the computation layer instead of using an
out-parameter, making "value not available" explicit and impossible to ignore:

- RigWeightedMeanCalc::weightedMeanOverCells
- RigStatisticsCalculator::mobileVolumeWeightedMean and all overrides

Time steps without a computable mean no longer contribute to the accumulated
sum, and are excluded from the divisor. Previously they would either add
garbage or, with a naive zero initialisation, bias the average towards zero.

RigStatisticsDataCache keeps the existing HUGE_VAL marker on its public API
via value_or(), so behaviour seen by RigHistogramData and the overlay info
is unchanged apart from the fix.

Also:
- Set m_isVolumeWeightedMeanCalculated in the per time step cache, which was
  never assigned, so the cached value was recomputed on every call.
- Remove a duplicated ensureKnownResultLoaded() call in
  RigEclipseNativeVisibleCellsStatCalc.

Fixes #14766

ApplicationLibCode/ResultStatisticsCache/RigStatisticsDataCache.cpp

@@ -331,7 +331,11 @@ void RigStatisticsDataCache::mobileVolumeWeightedMean( size_t timeStepIndex, dou
 
     if ( !m_statsPrTs[timeStepIndex].m_isVolumeWeightedMeanCalculated )
     {
-        m_statisticsCalculator->mobileVolumeWeightedMean( timeStepIndex, m_statsPrTs[timeStepIndex].m_volumeWeightedMean );
+        // HUGE_VAL is the "not available" marker used by the public statistics API
+        m_statsPrTs[timeStepIndex].m_volumeWeightedMean =
+            m_statisticsCalculator->mobileVolumeWeightedMean( timeStepIndex ).value_or( HUGE_VAL );
+
+        m_statsPrTs[timeStepIndex].m_isVolumeWeightedMeanCalculated = true;
     }
 
     mean = m_statsPrTs[timeStepIndex].m_volumeWeightedMean;
@@ -344,7 +348,7 @@ void RigStatisticsDataCache::mobileVolumeWeightedMean( double& mean )
 {
     if ( !m_statsAllTimesteps.m_isVolumeWeightedMeanCalculated )
     {
-        m_statisticsCalculator->mobileVolumeWeightedMean( m_statsAllTimesteps.m_volumeWeightedMean );
+        m_statsAllTimesteps.m_volumeWeightedMean = m_statisticsCalculator->mobileVolumeWeightedMean().value_or( HUGE_VAL );
 
         m_statsAllTimesteps.m_isVolumeWeightedMeanCalculated = true;
     }