Passphrase-locked vault still shows the pdv_file "Save to my vault" checkbox

On a Webform using the PdvRecordMap handler, when the owner's vault is passphrase-locked the form correctly shows the "Unlock my vault" prompt and hides the record store-consent checkboxes. But a pdv_file element's "Save or update this file in my vault for reuse" checkbox is still rendered, even though a write cannot succeed while the vault is locked.

Steps to reproduce

1. Enable an owner passphrase and lock the vault.
2. Open a Webform that maps a pdv_file element with #offer_vault_save (for example the consent-required onboarding demo form).
3. The unlock prompt appears and the record store-consent boxes are hidden, but the file "Save ... for reuse" checkbox remains.

Cause

The pdv_file save controls (save_to_vault / save_kind) are composite children built in the element #process pass, which runs after the handler's alterForm. At the time hideSaveControls() runs they are not yet keys in the form array, so hiding them by key is a no-op. The record store-consent checkboxes are real top-level elements, so they hide correctly.

Fix

Clear #offer_vault_save on every pdv_file element on the locked and unreachable-vault paths. The composite reads that flag when it later builds its children (PdvFile::saveToVaultElement returns nothing when it is falsy), so the save controls are never emitted: the same lever that already self-hides them for an anonymous submitter. A kernel test covers the locked case.