Add smoke tests for standard JSON Logic operators

We test custom operators (fractional, sem_ver, starts_with, ends_with) but have no isolated coverage for standard JSON Logic operators. Adding thin smoke tests would catch regressions during library upgrades.

Approach

• New files: evaluator/gherkin/jsonlogic-operators.feature + evaluator/flags/jsonlogic-operators-flags.json
• Tag: @jsonlogic-operators
• Each flag wraps the operator in if/== to select a variant; assertions are on resolved variant values only
• One or two cases per operator; not exhaustive

Operators

• Equality: ==, !=
• Comparison: >, <, >=, <=
• Logic: and, or, !, !!
• Arithmetic: +, -, *, /, %
• String: cat, substr, in (substring)
• Array: in (membership)
• Control: if, missing, missing_some

Coercion cases

• "1" == 1 (string/number)
• "2" > 1 (string in comparison)
• !!"" vs !!"hello" (truthy/falsy)
• + "3" (unary plus coercion)

Start with the existing custom-operator coverage, then add the tagged cases in evaluator/gherkin/jsonlogic-operators.feature and matching flags in evaluator/flags/jsonlogic-operators-flags.json. Run the evaluator's Gherkin tests and confirm each listed standard operator, including the coercion cases, resolves to the expected variant values.