fix: full-form internal links follow the {{link:}} target rules (ruling P10)

One PostIdParser for {{link:}} and select_post; post_link_url_<type> needs a
living, readable, non-attachment target of exactly that type.

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

tests/Support/ComposeFixture.php

@@ -105,6 +105,16 @@ final class ComposeFixture
             {
                 return $id !== 666;
             }
+
+            /** Ruling P10: 840 is a post, 666 no link target, 667 a readable attachment (none), every other ID a page. */
+            public function linkTargetType(int $id): ?string
+            {
+                return match ($id) {
+                    666, 667 => null,
+                    840 => 'post',
+                    default => 'page',
+                };
+            }
         };
     }