fix: memoize the rababa diacritizer per config

A single global cache meant the first config built served every
later call — the missing-config error could never fire once any
rababa map had run (the CI sweep does), and two configs in one
process silently shared one diacritizer. Keyed per config;
reset_cache clears the map.

Author: ronaldtse

lib/interscript/stdlib/functions/rababa_adapter.rb

@@ -1,7 +1,7 @@
 class Interscript::Stdlib
   module Functions
     class RababaAdapter
-      @rababa_diacritizer = nil
+      @rababa_diacritizers = {}
       @mutex = Mutex.new
 
       class << self
@@ -18,15 +18,15 @@ def reverse(output, config: nil)
         end
 
         def reset_cache
-          @mutex.synchronize { @rababa_diacritizer = nil }
+          @mutex.synchronize { @rababa_diacritizers.clear }
         end
 
         private
 
         def diacritizer_for(config_key)
           require_rababa!
           @mutex.synchronize do
-            @rababa_diacritizer ||= build_diacritizer(config_key)
+            @rababa_diacritizers[config_key] ||= build_diacritizer(config_key)
           end
         end