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/measurements/MeasurementContainer.js

@@ -1,9 +1,10 @@
 const MeasurementBuilder = require('./MeasurementBuilder');
 const EventEmitter = require('events');
 const convertModule = require('../convert/index');
+const { POSITIONS } = require('../constants/positions');
 
 class MeasurementContainer {
-  constructor(options = {},logger) {
+  constructor(options = {},_logger) {
     this.emitter = new EventEmitter();
     this.measurements = {};
     this.windowSize = options.windowSize || 10; // Default window size
@@ -359,7 +360,7 @@ class MeasurementContainer {
 
 
   // Difference calculations between positions
-difference({ from = "downstream", to = "upstream", unit: requestedUnit } = {}) {
+difference({ from = POSITIONS.DOWNSTREAM, to = POSITIONS.UPSTREAM, unit: requestedUnit } = {}) {
   if (!this._currentType || !this._currentVariant) {
     throw new Error("Type and variant must be specified for difference calculation");
   }
@@ -526,11 +527,11 @@ difference({ from = "downstream", to = "upstream", unit: requestedUnit } = {}) {
 
   _convertPositionStr2Num(positionString) {
     switch(positionString) {
-      case "atEquipment":
+      case POSITIONS.AT_EQUIPMENT:
         return 0;
-      case "upstream":
+      case POSITIONS.UPSTREAM:
         return Number.POSITIVE_INFINITY;
-      case "downstream":
+      case POSITIONS.DOWNSTREAM:
         return Number.NEGATIVE_INFINITY;
       
       default:
@@ -544,11 +545,11 @@ difference({ from = "downstream", to = "upstream", unit: requestedUnit } = {}) {
   _convertPositionNum2Str(positionValue) {
     switch (positionValue) {
     case 0:
-      return "atEquipment";
+      return POSITIONS.AT_EQUIPMENT;
     case (positionValue < 0):
-      return "upstream";
+      return POSITIONS.UPSTREAM;
     case (positionValue > 0):
-      return "downstream";
+      return POSITIONS.DOWNSTREAM;
     default:
       console.log(`Invalid position provided: ${positionValue}`);
     }