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/convert/index.js

@@ -249,7 +249,7 @@ Converter.prototype.list = function (measure) {
 Converter.prototype.throwUnsupportedUnitError = function (what) {
   var validUnits = [];
 
-  each(measures, function (systems, measure) {
+  each(measures, function (systems, _measure) {
     each(systems, function (units, system) {
       if(system == '_anchors')
         return false;