testing: add unit tests for metric-engine.ts and rule-engine.ts

[boldsbrainai]

Summary

metric-engine.ts and rule-engine.ts are the core evaluation engines of the product. They are currently only exercised indirectly through antipatterns-e2e.test.ts. Regressions in these engines could silently corrupt all analysis output without any test failure.

Scope

• src/core/metric-engine.test.ts — cover evaluate(), calibrate(), custom threshold overrides, edge cases (missing data, zero values)
• src/core/rule-engine.test.ts — cover compile(), condition evaluation, multi-condition rules, fallback behavior

Effort estimate

~1 day