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>

tests/LinkFilterTest.php

@@ -44,6 +44,7 @@ final class LinkFilterTest extends TestCase
     {
         $_SERVER['REQUEST_URI'] = '/' . $town_slug . '/';
         $_SERVER['HTTP_HOST'] = $host;
+        \Brain\Monkey\Functions\stubs(['did_action' => 1]);
         \Brain\Monkey\Functions\when('get_terms')->justReturn([11 => $town_slug]);
         \Brain\Monkey\Functions\when('get_field')->alias(function ($name) use ($main) {
             if ($name === 'main_town_slug') return $main;
@@ -62,6 +63,7 @@ final class LinkFilterTest extends TestCase
     {
         $_SERVER['REQUEST_URI'] = '/random/';
         $_SERVER['HTTP_HOST'] = 'site.com';
+        \Brain\Monkey\Functions\stubs(['did_action' => 1]);
         \Brain\Monkey\Functions\when('get_terms')->justReturn([10 => 'spb']);
         \Brain\Monkey\Functions\when('get_field')->justReturn(false);
 
@@ -129,6 +131,7 @@ final class LinkFilterTest extends TestCase
     {
         $_SERVER['REQUEST_URI'] = '/tyumen/';
         $_SERVER['HTTP_HOST'] = 'site.com';
+        \Brain\Monkey\Functions\stubs(['did_action' => 1]);
         \Brain\Monkey\Functions\when('get_terms')->justReturn([11 => 'tyumen']);
         \Brain\Monkey\Functions\when('get_field')->alias(function ($name) {
             if ($name === 'main_town_slug') return 'spb';