Replace deprecated reshape2::melt() with base R and tidyr equivalents

[utkarshpawade]

Fixes #478

Summary

• Replace all 9 uses of reshape2::melt() across 5 source files with
  base::as.data.frame.table(), tidyr::pivot_longer(), and explicit
  dplyr::bind_rows() constructions
• Remove reshape2 from DESCRIPTION Imports — it is no longer needed
• Update 10 vdiffr visual snapshots for ppd_stat plots affected by the migration
• Update inline documentation to remove references to reshape2::melt()

[codecov-commenter]

Codecov Report

❌ Patch coverage is 81.48148% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.55%. Comparing base (906cd5c) to head (ee24593).

Files with missing lines	Patch %	Lines
R/bayesplot-extractors.R	66.66%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #479      +/-   ##
==========================================
- Coverage   98.66%   98.55%   -0.12%     
==========================================
  Files          35       35              
  Lines        5860     5888      +28     
==========================================
+ Hits         5782     5803      +21     
- Misses         78       85       +7     

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[utkarshpawade]

@jgabry The 10 uncovered lines are in R/bayesplot-extractors.R, specifically the log_posterior.CmdStanMCMC and nuts_params.CmdStanMCMC methods which require a CmdStan installation to test. These lines were already uncovered before this PR, I just replaced uncovered reshape2 code with equivalent uncovered base R code. No regression in coverage.