Fix legacy caching module distribution for CI

The legacy caching bundle at sources/legacy-caching/dist/ was
gitignored by sources/.gitignore, so CI never included it. Fix by:
- Copying the bundle into sources/dist/legacy-caching/ during build
- Updating the loader path to resolve from dist/legacy-caching/
- Adding legacy module build steps to build-dist action and
  ci-update-dist workflow

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

build

@@ -6,6 +6,9 @@ build_legacy_caching() {
     npm clean-install
     npm run build
     cd ../..
+    # Copy legacy caching bundle into sources/dist so the CI bot picks it up
+    mkdir -p sources/dist/legacy-caching
+    cp sources/legacy-caching/dist/index.js sources/dist/legacy-caching/
 }
 
 cd sources