Use geom_step() as default in ppc_ecdf_overlay() and deprecate discrete argument

[utkarshpawade]

Fixes #259

The ECDF is a step function by definition, regardless of whether the data is continuous or discrete. ppc_km_overlay() already hard-codes geom_step() for the same reason. This PR makes ppc_ecdf_overlay() consistent.

Changes

• ppc_ecdf_overlay(), ppc_ecdf_overlay_grouped(), and ppd_ecdf_overlay() now always use geom = "step" in stat_ecdf()
• The discrete argument is deprecated via lifecycle::deprecate_warn()
• Updated roxygen documentation and examples
• Regenerated vdiffr snapshots
• Added deprecation and no-warning tests

[Copilot]

Pull request overview

This PR updates ECDF overlay plots to always render as step functions (consistent with the ECDF definition) and deprecates the discrete argument across related PPC/PPD helpers.

Changes:

• Hard-code geom = "step" in ppc_ecdf_overlay(), ppc_ecdf_overlay_grouped(), and ppd_ecdf_overlay().
• Deprecate the discrete argument using lifecycle::deprecate_warn().
• Update tests and vdiffr snapshots to reflect the new default rendering and to assert deprecation behavior.

Reviewed changes

Copilot reviewed 4 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
R/ppc-distributions.R	Switch ECDF overlays to step geom unconditionally; add deprecation warning and doc updates.
R/ppd-distributions.R	Switch PPD ECDF overlay to step geom unconditionally; add deprecation warning.
tests/testthat/test-ppc-distributions.R	Update vdiffr labels and add tests for deprecation + no-warning behavior when discrete isn’t supplied.
tests/testthat/_snaps/ppc-distributions/ppc-ecdf-overlay-default.svg	Regenerate snapshot for step-function ECDF rendering.
tests/testthat/_snaps/ppc-distributions/ppc-ecdf-overlay-size-alpha.svg	Regenerate snapshot and update title text.
tests/testthat/_snaps/ppc-distributions/ppc-ecdf-overlay-grouped-default.svg	Regenerate snapshot for grouped step-function ECDF rendering.
tests/testthat/_snaps/ppc-distributions/ppc-ecdf-overlay-grouped-size-alpha.svg	Regenerate snapshot and update title text.
NEWS.md	Document the behavior change and discrete deprecation.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.15%. Comparing base (83ad3ef) to head (c5a85b7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #533      +/-   ##
==========================================
- Coverage   99.18%   99.15%   -0.04%     
==========================================
  Files          35       35              
  Lines        5903     5920      +17     
==========================================
+ Hits         5855     5870      +15     
- Misses         48       50       +2     

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[jgabry]

Thanks, I agree it makes sense to just always use geom_step