Avoid redundant getPages() calls, which re-evaluate page #states when wizard_progress_states is on

Problem/Motivation

When wizard_progress_states is enabled, Webform::getPages($operation, $webform_submission) re-evaluates every page's #states through webform_submission.conditions_validator on every call. Webform doesn't cache that result.

The module calls it more often than it needs to:

• getCurrentPage() (src/WebformNavigationHelper.php:144-147) calls getPages() even when $webform_submission->getCurrentPage() is already set. The result is only used as a fallback.
• hasAccessToPage() (:490-494) calls getPages() on every call, including inside the validateAllPages() loop (:408) and the error loop in WebformNavigationHandler::validateForm().
• alterForm() and both progress preprocess hooks each call it again.

Proposed resolution

• In getCurrentPage(), return the submission's current page directly when it is set, and only call getPages() as the fallback.
• In hasAccessToPage(), and in loops that call it, compute the accessible page keys once per submission per request and reuse them. This could share the memo proposed in the "memoize getErrors()/hasVisitedPage()" issue.

Remaining tasks

• Write the MR.
• Check that the cached page list is refreshed when values change during the same request, since conditional pages depend on submission data.

User interface changes

None.

API changes

None.

Data model changes

None.