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-message-formatter.php

@@ -24,7 +24,8 @@ final class MessageFormatter
      *   reason:string,
      *   ip:string,
      *   user_agent:string,
-     *   is_retro:bool
+     *   is_retro:bool,
+     *   dry_run_drop?:bool
      * } $ctx
      *
      * @return array{text:string,reply_markup:array|null}