feat(utils): split-aware gamedata loader with tiered merge

Introduces loadGamedata(url, options?) and mergeGamedata(a, b) in
@nitrots/utils. The loader transparently accepts:

- a single-file URL (legacy) -> parsed as before
- a directory URL ending with '/' -> tier-merged from core/custom/seasonal,
  each tier driven by its own manifest.json5

Merge rules:

- arrays of objects sharing an id key (id, classname, name): merged by id,
  later layers overriding earlier ones
- arrays without an id key: concatenated
- plain objects: recursive merge per key
- anything else: later value wins

All gamedata consumers (FurnitureDataLoader, ProductDataLoader,
EffectAssetDownloadManager, AvatarRenderManager actions+figuredata,
LocalizationManager) are migrated to loadGamedata. Behaviour is unchanged
for single-file URLs, so existing deployments need no config changes;
opt-in to split mode by appending '/' to the URL once the layout is in
place.

README updated with the directory layout, merge table and programmatic
usage example. The companion CLI splitter that produces the core/ tier
from legacy files lives in the Nitro V3 client repo.

packages/localization/src/LocalizationManager.ts

@@ -1,7 +1,7 @@
 import { ILocalizationManager } from '@nitrots/api';
 import { BadgePointLimitsEvent, GetCommunication } from '@nitrots/communication';
 import { GetConfiguration } from '@nitrots/configuration';
-import { parseConfigJsonFromResponse } from '@nitrots/utils';
+import { loadGamedata } from '@nitrots/utils';
 import { BadgeBaseAndLevel } from './BadgeBaseAndLevel';
 
 export class LocalizationManager implements ILocalizationManager
@@ -26,28 +26,15 @@ export class LocalizationManager implements ILocalizationManager
 
                 url = GetConfiguration().interpolate(url);
 
-                let response: Response;
-
-                try
-                {
-                    response = await fetch(url);
-                }
-                catch(fetchErr)
-                {
-                    throw new Error(`Could not fetch localization file "${ url }" — check "external.texts.url" in ui-config.json (${ fetchErr.message })`);
-                }
-
-                if(response.status !== 200) throw new Error(`Failed to load localization file "${ url }" — server returned HTTP ${ response.status }. Check "external.texts.url" in ui-config.json`);
-
                 let data: any;
 
                 try
                 {
-                    data = await parseConfigJsonFromResponse(response, url);
+                    data = await loadGamedata(url);
                 }
-                catch(parseErr)
+                catch(err)
                 {
-                    throw new Error(`Invalid localization file "${ url }" — JSON/JSON5 parse failed. Check "external.texts.url" in ui-config.json (${ parseErr.message })`);
+                    throw new Error(`Could not load localization file "${ url }" — check "external.texts.url" in ui-config.json (${ err?.message || err })`);
                 }
 
                 this.parseLocalization(data);