feat(web): coalesce existing-cart refreshes

[rsomlette]

Review purpose

Add the existing-cart refresh signal as a separate, reviewable behavior instead of folding it into the general element lifecycle state machine.

Stacked on wallets/release-04-lifecycle.

Changes

• add the public cartUpdated() method for an active existing-cart flow
• forward a refresh request through the private adapter boundary
• coalesce repeated signals from the same synchronous turn
• preserve one additional signal received while a refresh is running
• hold a signal received during initialization until the flow is ready
• discard queued work when the configured cart is replaced
• ignore product-flow and disconnected-element signals
• normalize refresh failures without exposing private exception messages
• allow a later merchant signal to retry after a failed refresh

Non-goals

• fetching or replacing the merchant cart in Checkout Kit
• defining PW policy for a cart mutation that overlaps an active wallet attempt
• a real Portable Wallets adapter
• payment or completion retries

Related ticket

Related: https://github.com/shop/issues-checkout/issues/14707

This PR satisfies the Kit-side queueing and forwarding contract against the fake adapter. It does not close the ticket until the real PW adapter consumes the signal and the active-attempt policy is verified.

Validation

• web lint and typecheck
• 280 web tests
• production package build
• multi-page sample build
• publint
• package snapshot comparison