fix: wire is_hard_drop_enabled + drop_score_threshold settings into Classifier filter chain

Settings::register_classifier_filters() registers priority-5 callbacks on
cf7stg_classifier_thresholds and cf7stg_hard_drop_rules so the UI options
actually influence Classifier behaviour. Called from Plugin::boot(). User
filters at priority 10 override these defaults as expected. Adds 4 tests
covering the wired behaviour and override priority. Also adds dry_run_drop?
to MessageFormatter::build() @param docblock.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

includes/class-plugin.php

@@ -43,6 +43,9 @@ final class Plugin
 
     public function boot(): void
     {
+        // Inject silent-drop options into Classifier filter chain
+        Settings::register_classifier_filters();
+
         // Register runtime hooks
         Dispatcher::register_hooks();
         (new CF7Source())->register();