excore check: report rule coverage and warn about rules with no effect

[Tet-9]

Rules are applied in order, so a later rule can silently override everything an earlier one matched. The earlier rule then looks active in the manifest but changes nothing, which is an easy mistake to make when tuning recipes.

excore check now prints how many units stay on the default and warns (stderr, exit code unchanged) for every rule that ends up owning no units. ScreenReport gains rule_units, default_units and unused_rules. Tests cover the counts and the CLI warning.

[ikeige-coder]

Merging. This catches rules that silently own no units after later rules override them, which is an easy mistake when tuning recipes. It's a warning only, exit code unchanged, and the tests cover the counts and the CLI output.
Follow-up nit, not blocking: a rule can also own zero units because its match hits nothing (a typo). The warning currently says "later rules override every unit it matches". A reword would actually do, But nevermind.