mcmc_areas_data() processes MCMC array three times redundantly

[utkarshpawade]

mcmc_areas_data() calls mcmc_intervals_data() at line 694, which internally runs prepare_mcmc_array() + merge_chains() + melt_mcmc(). Immediately after, mcmc_areas_data() calls prepare_mcmc_array() and merge_chains() again (lines 698-699) to compute density curves.

This means the same array preparation and chain merging happens three times per call. For models with many parameters or long chains, this adds measurable overhead.