fix: bounded usage scan and count-only unreadable posts in global edits (L1, L4)

GlobalUsages runs one query per request (ID, status, type – no content),
checks read_post/edit_post in chunks of 200 with a primed and released post
cache, lists at most 50 readable posts and refuses a confirmed global edit
above 2000 live references (unsupported, edit in the Divi builder). Posts the
caller may not read appear only as hidden_count / blocked_hidden_count;
blocked_ids names readable posts only.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/Integrations/Divi/Pages/DiviWriteAbility.php

@@ -39,6 +39,7 @@ abstract class DiviWriteAbility implements Ability
             // made when revisions are off.
             'affected' => ['type' => 'array', 'items' => ['type' => 'object']],
             'affected_count' => ['type' => 'integer'],
+            'hidden_count' => ['type' => 'integer'],
             'backup_id' => ['type' => 'integer'],
         ], ['tree_rev']);
     }