Unify density-control argument defaults

[utkarshpawade]

Fixes #131
Unifies density-control defaults across the ppc_* / ppd_* density-overlay functions with the mcmc_areas* / mcmc_dens* family.

• ppc_dens_overlay(), ppc_dens_overlay_grouped(), ppd_dens_overlay(): bw, adjust, kernel, n_dens defaults changed from hard-coded values to NULL, with %||% fallbacks applied internally.
• compute_interval_density(): %||% fallbacks extended to bw/adjust/kernel.
• args-density-controls template: documents all four defaults.
• Regenerated .Rd files.

Behavior is preserved. NULL resolves to the previously hard-coded values.

[Copilot]

Pull request overview

This PR standardizes how density-control arguments (bw, adjust, kernel, n_dens) are defaulted across PPC/PPD density-overlay functions to match the broader “density-like” plotting API, by switching user-facing defaults to NULL and applying internal %||% fallbacks to preserve existing behavior.

Changes:

• Updated ppc_dens_overlay(), ppc_dens_overlay_grouped(), and ppd_dens_overlay() to default density controls to NULL and set effective defaults internally.
• Extended compute_interval_density() to apply %||% fallbacks for bw/adjust/kernel.
• Updated shared roxygen template + regenerated .Rd files; added a NEWS entry.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
R/ppd-distributions.R	Switches ppd_dens_overlay() density-control defaults to NULL with internal fallbacks.
R/ppc-distributions.R	Switches ppc_dens_overlay*() density-control defaults to NULL with internal fallbacks (via ppc_dens_overlay()).
R/mcmc-intervals.R	Adds `%
man-roxygen/args-density-controls.R	Updates the shared parameter documentation for density controls.
man/PPD-distributions.Rd	Regenerated docs reflecting the updated defaults and parameter text.
man/PPC-distributions.Rd	Regenerated docs reflecting the updated defaults and parameter text.
man/MCMC-intervals.Rd	Regenerated docs reflecting the updated density-controls template text.
man/MCMC-distributions.Rd	Regenerated docs reflecting the updated density-controls template text.
NEWS.md	Notes the change in defaults to NULL for affected functions.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.15%. Comparing base (44c3e5c) to head (8012e75).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #538   +/-   ##
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  Coverage   99.15%   99.15%           
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  Files          35       35           
  Lines        5920     5931   +11     
=======================================
+ Hits         5870     5881   +11     
  Misses         50       50           

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