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/avatar/src/EffectAssetDownloadManager.ts

@@ -1,7 +1,7 @@
 import { IAssetManager, IAvatarEffectListener } from '@nitrots/api';
 import { GetConfiguration } from '@nitrots/configuration';
 import { AvatarRenderEffectLibraryEvent, GetEventDispatcher, NitroEvent, NitroEventType } from '@nitrots/events';
-import { parseConfigJsonFromResponse } from '@nitrots/utils';
+import { loadGamedata } from '@nitrots/utils';
 import { AvatarStructure } from './AvatarStructure';
 import { EffectAssetDownloadLibrary } from './EffectAssetDownloadLibrary';
 
@@ -32,28 +32,15 @@ export class EffectAssetDownloadManager
 
         if(!url || !url.length) throw new Error('Missing "avatar.effectmap.url" in config — add the effect map URL to your renderer-config.json');
 
-        let response: Response;
-
-        try
-        {
-            response = await fetch(url);
-        }
-        catch(fetchErr)
-        {
-            throw new Error(`Could not fetch effect map from "${ url }" — check "avatar.effectmap.url" in renderer-config.json (${ fetchErr.message })`);
-        }
-
-        if(response.status !== 200) throw new Error(`Failed to load effect map from "${ url }" — server returned HTTP ${ response.status }. Check "avatar.effectmap.url" in renderer-config.json`);
-
         let responseData: any;
 
         try
         {
-            responseData = await parseConfigJsonFromResponse(response, url);
+            responseData = await loadGamedata(url);
         }
-        catch(parseErr)
+        catch(err)
         {
-            throw new Error(`Invalid effect map "${ url }" — JSON/JSON5 parse failed. Check "avatar.effectmap.url" in renderer-config.json (${ parseErr.message })`);
+            throw new Error(`Could not load effect map from "${ url }" — check "avatar.effectmap.url" in renderer-config.json (${ err?.message || err })`);
         }
 
         this.processEffectMap(responseData.effects);