fix(membership): retroactive wizard assigns wrong person_id to dependents

JS form sends 1-based person_index (child_1, child_2) but PHP used it
directly as a 0-based array subscript into personIdMap, causing child_1
to get child_2's ID and child_2 to fall through to name-based resolution
(which could fail or duplicate). Convert to 0-based before array lookup.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Modules/Members/Services/RetroactiveMembershipService.php

@@ -384,14 +384,18 @@ final class RetroactiveMembershipService
 
                     if ($personType === 'member') {
                         $personId = $memberId;
-                    } elseif (isset($personIdMap[$personType][$personIndex])) {
-                        $personId = $personIdMap[$personType][$personIndex];
+                    } else {
+                        // JS sends 1-based indices (child_1, child_2), convert to 0-based array index
+                        $arrayIndex = max(0, $personIndex - 1);
+                        if (isset($personIdMap[$personType][$arrayIndex])) {
+                            $personId = $personIdMap[$personType][$arrayIndex];
                         } else {
                             $personId = self::resolveDependentId($db, $memberId, $personType, $sub['person_name'] ?? '');
                             if ($personId === null) {
                                 continue;
                             }
                         }
+                    }
 
                     $sub['person_id'] = $personId;