Add QML resource replacement support for specific files

Implements a mechanism to replace specific QML resource files at runtime by reading replacement files from a designated directory. Updates the resource registration hook to rebuild resource data tables when replacements are present, and adds utility functions and structures for managing replacement entries. Only selected files are eligible for replacement, and the README is updated with instructions for using this feature.

src/reMarkable/reMarkable.m

@@ -442,33 +442,90 @@ extern "C" int hooked_qRegisterResourceData(
     }
 
     pthread_mutex_lock(&gResourceMutex);
-
     struct ResourceRoot resource = {
         .data = (uint8_t *)data,
         .name = (uint8_t *)name,
         .tree = (uint8_t *)tree,
+
         .treeSize = 0,
         .dataSize = 0,
         .originalDataSize = 0,
         .nameSize = 0,
+
         .entriesAffected = 0,
     };
 
+    NSLogger(@"[reMarkable] Registering Qt resource version %d tree:%p name:%p data:%p",
+             version, tree, name, data);
+
     statArchive(&resource, 0);
-    processNode(&resource, 0, "");
+    
+    // Make a writable copy of the tree (we need to modify offsets)
     resource.tree = (uint8_t *)malloc(resource.treeSize);
-    if (resource.tree) {
-        memcpy(resource.tree, tree, resource.treeSize);
+    if (!resource.tree) {
+        NSLogger(@"[reMarkable] Failed to allocate tree buffer");
+        pthread_mutex_unlock(&gResourceMutex);
+        return original_qRegisterResourceData(version, tree, name, data);
+    }
+    memcpy(resource.tree, tree, resource.treeSize);
+
+    // Process nodes and mark replacements
+    processNode(&resource, 0, "");
+    NSLogger(@"[reMarkable] Processing done! Entries affected: %d, dataSize: %zu, originalDataSize: %zu", 
+             resource.entriesAffected, resource.dataSize, resource.originalDataSize);
+
+    const unsigned char *finalTree = tree;
+    const unsigned char *finalData = data;
+    uint8_t *newDataBuffer = NULL;
+
+    if (resource.entriesAffected > 0) {
+        NSLogger(@"[reMarkable] Rebuilding data tables... (entries: %d)", resource.entriesAffected);
+        
+        // Allocate new data buffer (original size + space for replacements)
+        newDataBuffer = (uint8_t *)malloc(resource.dataSize);
+        if (!newDataBuffer) {
+            NSLogger(@"[reMarkable] Failed to allocate new data buffer (%zu bytes)", resource.dataSize);
+            free(resource.tree);
+            clearReplacementEntries();
+            pthread_mutex_unlock(&gResourceMutex);
+            return original_qRegisterResourceData(version, tree, name, data);
+        }
+        
+        // Copy original data
+        memcpy(newDataBuffer, data, resource.originalDataSize);
+        
+        // Copy replacement entries to their designated offsets
+        struct ReplacementEntry *entry = getReplacementEntries();
+        while (entry) {
+            // Write size prefix (4 bytes, big-endian)
+            writeUint32(newDataBuffer, (int)entry->copyToOffset, (uint32_t)entry->size);
+            // Write data after size prefix
+            memcpy(newDataBuffer + entry->copyToOffset + 4, entry->data, entry->size);
+            
+            NSLogger(@"[reMarkable] Copied replacement for node %d at offset %zu (%zu bytes)", 
+                     entry->node, entry->copyToOffset, entry->size);
+            
+            entry = entry->next;
+        }
+        
+        finalTree = resource.tree;
+        finalData = newDataBuffer;
+        
+        NSLogger(@"[reMarkable] Data buffer rebuilt: original %zu bytes -> new %zu bytes", 
+                 resource.originalDataSize, resource.dataSize);
     }
 
-    NSLogger(@"[reMarkable] Registering Qt resource version %d tree:%p (size:%zu) name:%p (size:%zu) data:%p (size:%zu)",
-             version, tree, resource.treeSize, name, resource.nameSize, data, resource.dataSize);
-
-    int status = original_qRegisterResourceData(version, tree, name, data);
-    pthread_mutex_unlock(&gResourceMutex);
-    if (resource.tree) {
+    int status = original_qRegisterResourceData(version, finalTree, name, finalData);
+    
+    // Cleanup
+    clearReplacementEntries();
+    if (resource.tree && resource.entriesAffected == 0) {
         free(resource.tree);
     }
+    // Note: We intentionally don't free newDataBuffer or resource.tree when entriesAffected > 0
+    // because Qt will use these buffers for the lifetime of the application
+    
+    pthread_mutex_unlock(&gResourceMutex);
     return status;
 }
 #endif // BUILD_MODE_QMLDIFF