Fix: Add missing Handlebars subtract helper

- Added subtract helper: parseInt(a) - parseInt(b)
- Helper is used in partial-active-effects.hbs and partial-item-effects.hbs
- Registered in MournbladeCYD2Utility.init() alongside other helpers
- Added test verification for subtract helper registration

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

modules/mournblade-cyd2-utility.js

@@ -46,6 +46,9 @@ export class MournbladeCYD2Utility {
     Handlebars.registerHelper('mul', function (a, b) {
       return parseInt(a) * parseInt(b);
     })
+    Handlebars.registerHelper('subtract', function (a, b) {
+      return parseInt(a) - parseInt(b);
+    })
     Handlebars.registerHelper('select', function(value, options) {
       const html = options.fn(this);
       const escaped = String(value).replace(/[.*+?^${}()|[\]\\]/g, '\\$&');