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.downloader.ts

@@ -81,7 +81,7 @@ export class SchematronDownloader {
   private cacheDir: string;
   private smartfile: any;
   
-  constructor(cacheDir: string = 'assets/schematron') {
+  constructor(cacheDir: string = 'assets_downloaded/schematron') {
     this.cacheDir = cacheDir;
   }
   
@@ -278,7 +278,7 @@ export const ISO_SCHEMATRON_SKELETONS = {
 /**
  * Download ISO Schematron skeleton files
  */
-export async function downloadISOSkeletons(targetDir: string = 'assets/schematron/iso'): Promise<void> {
+export async function downloadISOSkeletons(targetDir: string = 'assets_downloaded/schematron/iso'): Promise<void> {
   await fs.mkdir(targetDir, { recursive: true });
   
   console.log('Downloading ISO Schematron skeleton files...');