fix(routing): bootstrap permalink structure so fresh installs resolve single posts

A fresh install leaves `permalink_structure` empty, so WordPress snapshots
$wp_rewrite with no post/page permalink rules and every single post resolves
to 404 until someone re-saves Settings -> Permalinks.

The existing mitigation fired too late to help: routing.php wrote the option
and flushed from `init`, but WP_Rewrite was already constructed with the empty
structure, and core defers pre-`wp_loaded` flushes to a `wp_loaded` callback
that regenerates from that stale snapshot. The rewrite_rules option ended up
non-empty but broken, so the 404 persisted on every request.

New \Jio\Core\RewriteBootstrap owns this: on `after_setup_theme` (still inside
wp-settings.php, before wp-blog-header.php parses the query) it calls
$wp_rewrite->set_permalink_structure() — the same API wp-admin/options-permalink.php
uses — which persists the option *and* refreshes the in-memory snapshot, so both
the current request and the rules written for the next one are correct. A
structure an administrator chose is never overridden.

Also consolidates the two duplicated flush sites that lived inline in
functions.php (`after_switch_theme` closure and the `jio_flushed_version`
block, whose `delete_option('rewrite_rules')` forced a needless regenerate),
and drops the now-redundant permalink write from routing.php. The version
stamp moves to `wp_loaded` priority 20 so it sees the complete rule set
instead of the pre-`add_rewrite_rule()` snapshot.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

inc/modules/user-parts/routing.php

@@ -189,20 +189,14 @@ add_action('init', function () {
     }
 
     // 统一 flush 检查 — bump when user-home rewrite / author override changes
+    // 固定链接结构的兜底写入由 \Jio\Core\RewriteBootstrap 负责（after_setup_theme），
+    // 此处只关心本模块自己的规则版本与规则缺失。
 	$rewrite_version = JIO_VERSION . ':message-routing:' . jio_get_messages_rewrite_slug();
     $flushed = get_option('jio_user_rules_flushed');
     $rules   = get_option('rewrite_rules');
     $need_flush = ($flushed !== $rewrite_version)
         || !is_array($rules)
-        || $rules === []
-        || !get_option('permalink_structure');
-
-    // Dev/plain installs often have empty permalink_structure → pretty /user/ 404s at Apache.
-    // Force a usable structure so registered rewrite rules actually apply.
-    if (!get_option('permalink_structure')) {
-        update_option('permalink_structure', '/%postname%/');
-        $need_flush = true;
-    }
+        || $rules === [];
 
     if ($need_flush) {
         flush_rewrite_rules(true);