Add per-expansion asset overlay system and fix CharSections DBC layout

Expansion overlays allow each expansion to supplement the base asset data
via an assetManifest field in expansion.json, loaded at priority 50 (below
HD packs). The asset extractor gains --reference-manifest for delta-only
extraction. Also fixes CharSections field indices (VariationIndex=4,
ColorIndex=5, Texture1=6) across all DBC layout references.

tools/asset_extract/extractor.cpp

@@ -246,6 +246,58 @@ static std::unordered_set<std::string> buildWantedDbcSet(const Extractor::Option
     return wanted;
 }
 
+// Load all entry keys from a manifest.json into a set of normalized WoW paths.
+// This is a minimal parser — just extracts the keys from the "entries" object
+// without pulling in a full JSON library.
+static std::unordered_set<std::string> loadManifestKeys(const std::string& manifestPath) {
+    std::unordered_set<std::string> keys;
+    std::ifstream f(manifestPath);
+    if (!f.is_open()) {
+        std::cerr << "Failed to open reference manifest: " << manifestPath << "\n";
+        return keys;
+    }
+
+    // Find the "entries" section, then extract keys from each line
+    bool inEntries = false;
+    std::string line;
+    while (std::getline(f, line)) {
+        if (!inEntries) {
+            if (line.find("\"entries\"") != std::string::npos) {
+                inEntries = true;
+            }
+            continue;
+        }
+
+        // End of entries block
+        size_t closeBrace = line.find_first_not_of(" \t");
+        if (closeBrace != std::string::npos && line[closeBrace] == '}') {
+            break;
+        }
+
+        // Extract key: find first quoted string on the line
+        size_t q1 = line.find('"');
+        if (q1 == std::string::npos) continue;
+        size_t q2 = q1 + 1;
+        // Find closing quote (handle escaped backslashes)
+        std::string key;
+        while (q2 < line.size() && line[q2] != '"') {
+            if (line[q2] == '\\' && q2 + 1 < line.size()) {
+                key += line[q2 + 1];  // unescape \\, \", etc.
+                q2 += 2;
+            } else {
+                key += line[q2];
+                q2++;
+            }
+        }
+
+        if (!key.empty()) {
+            keys.insert(key);  // Already normalized (lowercase, backslashes)
+        }
+    }
+
+    return keys;
+}
+
 // Known WoW client locales
 static const std::vector<std::string> kKnownLocales = {
     "enUS", "enGB", "deDE", "frFR", "esES", "esMX",
@@ -485,6 +537,22 @@ bool Extractor::run(const Options& opts) {
         return false;
     }
 
+    // Delta extraction: filter out files that already exist in the reference manifest
+    if (!opts.referenceManifest.empty()) {
+        auto refKeys = loadManifestKeys(opts.referenceManifest);
+        if (refKeys.empty()) {
+            std::cerr << "Warning: reference manifest is empty or failed to load\n";
+        } else {
+            size_t before = files.size();
+            files.erase(std::remove_if(files.begin(), files.end(),
+                [&refKeys](const std::string& wowPath) {
+                    return refKeys.count(normalizeWowPath(wowPath)) > 0;
+                }), files.end());
+            std::cout << "Delta filter: " << before << " -> " << files.size()
+                      << " files (" << (before - files.size()) << " already in reference)\n";
+        }
+    }
+
     if (files.empty()) {
         std::cerr << "No files to extract\n";
         return false;