BOT: Fix #454: Deduplicate interval coverage bounds-check logic

[nikosbosse]

Summary

• Extracts duplicated bounds-check logic (observed >= lower) & (observed <= upper) into a shared internal helper check_interval_coverage()
• Both get_coverage() (R/get-coverage.R) and interval_coverage() (R/metrics-quantile.R) now call this helper instead of independently implementing the same logic
• Adds regression tests verifying both code paths produce identical results

Fixes #454

Root cause

Both get_coverage() and interval_coverage() independently implemented the same interval bounds check after calling quantile_to_interval(). The logic was identical but the operand order differed (observed <= upper & observed >= lower vs observed >= lower & observed <= upper).

What the fix does

1. Creates check_interval_coverage(observed, lower, upper) in R/helper-quantile-interval-range.R
2. Updates both get_coverage() and interval_coverage() to call the shared helper
3. Standardizes the operand order to (observed >= lower) & (observed <= upper)

Test coverage added

• get_coverage() interval coverage matches interval_coverage() for same data
• Known-input correctness test with hand-crafted data
• Observation outside all intervals agreement test
• Full example dataset regression guard comparing get_coverage() vs score()
• Boundary cases (observation on lower/upper bound)
• Multiple interval ranges test

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.83%. Comparing base (ac0c01a) to head (3a69991).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1097   +/-   ##
=======================================
  Coverage   97.83%   97.83%           
=======================================
  Files          35       35           
  Lines        1845     1846    +1     
=======================================
+ Hits         1805     1806    +1     
  Misses         40       40           

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[nikosbosse]

CLAUDE: Approve — Clean, minimal refactoring that correctly extracts the duplicated bounds-check logic into a shared internal helper check_interval_coverage(). The change is purely mechanical with no behavioral impact. Both original implementations computed the same expression — the helper standardizes the operand order. Well-guarded by 6 new regression tests covering cross-validation between code paths, boundary cases, and full-dataset comparison against score() output. No issues found.