Stop tracking the FR2 editor cache

Remove Assets/FR2_Cache.asset (+ .meta) and gitignore it - it is a
regenerated Find References 2 editor tool cache, not source.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -85,3 +85,5 @@ crashlytics-build.properties
 
 # Project mirrors
 /Mirror/
+
+/Assets/FR2_Cache.asset