Fix cache manifest not saving: open MPQ writable in ProjectConfigBuilder

`ProjectConfigBuilder.apply()` opens the target map MPQ to save the
updated cache manifest, but passes `readOnly=true` to `getEditor()`.
The subsequent `saveManifest()` call tries to insert a file via
`mpq.insertFile()`, which calls `JMpqEditor.deleteFile()` internally.
JMPQ3 throws `NonWritableChannelException` (with a null message)
when the editor is read-only, resulting in:

  Warning: Could not save manifest to MPQ: null

This silently breaks the build caching system introduced in #1096 —
every build becomes a full rebuild because the manifest is never
persisted.

Fix: change `readOnly` from `true` to `false` so the manifest can
actually be written.

Author: Donach

de.peeeq.wurstscript/src/main/java/de/peeeq/wurstio/languageserver/ProjectConfigBuilder.java

@@ -97,8 +97,8 @@ public static MapRequest.CompilationResult apply(WurstProjectConfigData projectC
         // Apply map header (this is cheap, so we always do it)
         applyMapHeader(projectConfig, targetMap);
 
-        // Update the manifest with new config hash
-        try (MpqEditor mpq = MpqEditorFactory.getEditor(Optional.of(targetMap), true)) {
+        // Update the manifest with new config hash (must open writable to insert)
+        try (MpqEditor mpq = MpqEditorFactory.getEditor(Optional.of(targetMap), false)) {
             ImportFile.CacheManifest manifest = ImportFile.getCachedManifest(mpq).orElse(new ImportFile.CacheManifest());
             manifest.setMapConfig(configHash);
             ImportFile.saveManifest(mpq, manifest);