refactor: Move downloaded resources from assets/ to assets_downloaded/

- Changed default download location to assets_downloaded/schematron
- Updated all references in SchematronDownloader, integration, and validator
- Updated postinstall scripts to use new location
- assets_downloaded/ is already in .gitignore to exclude downloaded files from git
- Moved existing downloaded files to new location
- All functionality tested and working correctly

ts/formats/validation/schematron.integration.ts

@@ -75,7 +75,7 @@ export class IntegratedValidator {
     standard: 'EN16931' | 'PEPPOL' | 'XRECHNUNG',
     format: 'UBL' | 'CII'
   ): Promise<string | null> {
-    const basePath = 'assets/schematron';
+    const basePath = 'assets_downloaded/schematron';
     
     // Map standard and format to file pattern
     const patterns: Record<string, Record<string, string>> = {