fix(wpmc 1.0.1): guard ACF helpers with did_action('acf/init')

Add private Helpers::acf_ready() that checks did_action('acf/init') > 0,
then guard the five ACF-dependent methods (alt_get_field, index_town_alt_page_id,
main_town_slug, page_unic_swap, is_routable_path) so they return safe defaults
(null/true) before ACF initialises. Fixes the acf_get_value "called too early"
notice triggered when home_url filter fires before acf/init.

Update existing tests in HelpersTest, LinkFilterTest, RedirectTest, RouterTest
to stub did_action => 1 where ACF-ready behaviour is expected. Add four new
guard tests covering before/after acf/init for main_town_slug and is_routable_path.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

includes/class-helpers.php

@@ -11,6 +11,11 @@ final class Helpers
     private static array $page_unic_cache = [];
     private static int $bypass_depth = 0;
 
+    private static function acf_ready(): bool
+    {
+        return \did_action('acf/init') > 0;
+    }
+
     public static function reset_cache(): void
     {
         self::$current_town_memo = null;
@@ -80,6 +85,9 @@ final class Helpers
 
     public static function alt_get_field(string $selector, $post_id = false, bool $format_value = true)
     {
+        if (!self::acf_ready()) {
+            return null;
+        }
         $post_id = \apply_filters('wpmc_alt_get_field_post_id', $post_id, $selector);
         if (!\function_exists('get_field')) {
             return null;
@@ -106,6 +114,9 @@ final class Helpers
 
     public static function index_town_alt_page_id(): ?int
     {
+        if (!self::acf_ready()) {
+            return null;
+        }
         if (!\function_exists('get_field')) {
             return null;
         }
@@ -116,6 +127,9 @@ final class Helpers
 
     public static function main_town_slug(): ?string
     {
+        if (!self::acf_ready()) {
+            return null;
+        }
         if (!\function_exists('get_field')) {
             return null;
         }
@@ -132,6 +146,9 @@ final class Helpers
         if (array_key_exists($key, self::$page_unic_cache)) {
             return self::$page_unic_cache[$key];
         }
+        if (!self::acf_ready()) {
+            return null;
+        }
         $term = self::get_term_by_slug($town_slug);
         if ($term === null) {
             return self::$page_unic_cache[$key] = null;
@@ -161,6 +178,9 @@ final class Helpers
         if ($path === null) {
             $path = $_SERVER['REQUEST_URI'] ?? '/';
         }
+        if (!self::acf_ready()) {
+            return true;
+        }
         if (!\function_exists('get_field')) {
             return true;
         }