feat(engine): damage-resistance modifier descriptor

- Add DAMAGE_RESISTANCE_DESCRIPTOR with multiplicative stacking rule
- Export applyResistance() and stackResistances() math helpers
- 14 tests covering registry, apply, describe, applyResistance, stackResistances
- Fix eslint varsIgnorePattern to allow _-prefixed destructuring vars
- Remove now-redundant eslint-disable comment in schema.test.ts

eslint.config.js

@@ -8,7 +8,7 @@ export default tseslint.config(
   {
     rules: {
       "@typescript-eslint/no-explicit-any": "error",
-      "@typescript-eslint/no-unused-vars": ["error", { argsIgnorePattern: "^_" }],
+      "@typescript-eslint/no-unused-vars": ["error", { argsIgnorePattern: "^_", varsIgnorePattern: "^_" }],
     },
   },
   // Engine RHS purity: ban impure globals in rete/src