refactor: adopt POSITIONS constants, fix ESLint warnings, break menuUtils into modules

- Replace hardcoded position strings with POSITIONS.* constants
- Prefix unused variables with _ to resolve no-unused-vars warnings
- Fix no-prototype-builtins with Object.prototype.hasOwnProperty.call()
- Extract menuUtils.js (543 lines) into 6 focused modules under menu/
- menuUtils.js now 35 lines, delegates via prototype mixin pattern
- Add 158 unit tests for ConfigManager, MeasurementContainer, ValidationUtils

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/helper/childRegistrationUtils.js

@@ -5,7 +5,7 @@ class ChildRegistrationUtils {
     this.registeredChildren = new Map();
   }
 
-  async registerChild(child, positionVsParent, distance) {
+  async registerChild(child, positionVsParent, _distance) {
     const softwareType = (child.config.functionality.softwareType || '').toLowerCase();
     const { name, id } = child.config.general;