BOT: Fix #942: Add plot_discrimination() for binary forecasts

[nikosbosse]

Summary

• Adds a new plot_discrimination() function that visualises the discrimination ability of binary forecasts by plotting overlapping density curves of predicted probabilities, stratified by observed outcome level
• Includes 7 tests covering basic usage, faceting, plain data.frame input, missing columns, single-model data, separation verification, and edge cases
• Follows existing plot function conventions (ensure_data.table(), check_columns_present(), theme_scoringutils())

Closes #942

Root cause

No binary-specific plot functions existed in the codebase. The forecast_binary class and example_binary dataset provided the data structures but there was no way to visualise discrimination ability.

What changed

• New file R/plot-discrimination.R with plot_discrimination() — accepts a data.frame/data.table with observed (factor) and predicted (numeric) columns, renders overlapping density plots via ggplot2
• New test file tests/testthat/test-plot_discrimination.R with 7 test cases and 2 visual regression snapshots
• NAMESPACE updated to export the new function

Test plan

• All 7 new tests pass
• Full test suite passes (689 tests, 0 failures)
• R CMD check: 0 errors, 0 warnings, 2 notes (pre-existing)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.84%. Comparing base (ac0c01a) to head (cc0dbc2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1079      +/-   ##
==========================================
+ Coverage   97.83%   97.84%   +0.01%     
==========================================
  Files          35       36       +1     
  Lines        1845     1857      +12     
==========================================
+ Hits         1805     1817      +12     
  Misses         40       40              

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[nikosbosse]

CLAUDE: Approved. Clean implementation of plot_discrimination() that follows all existing package conventions (ensure_data.table, check_columns_present, theme_scoringutils). All 7 tests match specifications and pass. Only minor nitpick: unused @importFrom checkmate assert_data_frame import — harmless but unnecessary.