fix: keep the guest's contact email on the cart and order addresses

[alecritson]

Problem

A guest's email never reached the order. The contact step stores it on the checkout session (and on the shipping row if one already exists), but:

• Lunar rewrites an address row wholesale on setShippingAddress / setBillingAddress.
• The address forms never send an email, so toCartAddressData writes contact_email as empty, wiping whatever the contact step copied.
• assertContactKnown reads the session first, so payment still goes through and nothing flags the gap.

Result: placed guest orders have no contact_email on either address, so order confirmation mails and the panel have nobody to write to.

Fix

• Address writes (storeShippingAddress, storeBillingAddress, and the express collect mirror) carry the email over: the payload's own email wins (wallet sheets send one), then the existing row's, then the session's.
• Completion stamps any order address still missing an email from the session, falling back to the signed-in customer's email. Covers contact given after the last address write, and both the sync and async paths.

Tests

• New GuestContactEmailTest: contact then shipping address, billing address, payload email wins, and a guest order placed with both addresses carrying the email.
• ExpressCollectFlowTest: the basket page wallet collect order now asserts both order addresses carry the email (fails before the fix).
• Full suite: 272 passed, 1 skipped. phpstan and pint clean.

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