feat(web): add deterministic wallet element lifecycle

[rsomlette]

Review purpose

Implement one deterministic custom-element lifecycle state machine against a Kit-internal fake adapter. This keeps public lifecycle semantics reviewable without a World, Core, or Portable Wallets runtime dependency.

Stacked on wallets/release-03-merchant-contract.

Changes

• add a private adapter interface and injectable fake factory
• coalesce synchronous configuration writes
• avoid restarts for equal effective configuration
• cancel stale generations and ignore late outcomes
• expose loading, ready, unavailable, and normalized error mirrors
• notify callbacks first and dispatch equivalent composed DOM events
• isolate merchant callback exceptions
• cancel work on disconnect and restart cleanly on remount
• cover multiple instances, route-style updates, and pre-upgrade properties
• make server module evaluation safe when DOM globals are unavailable
• keep thrown adapter messages out of public errors

Non-goals

• a real Portable Wallets import or private runtime URL
• wallet configuration network requests
• cartUpdated() forwarding
• skeleton markup, styling, or transitions
• PW-specific error mapping
• payment, completion, recovery, or navigation

Related tickets

Primary: https://github.com/shop/issues-checkout/issues/14701

Related:

• https://github.com/shop/issues-checkout/issues/14700
• https://github.com/shop/issues-checkout/issues/14702
• https://github.com/shop/issues-checkout/issues/14704

The fake-adapter ticket remains open until the adapter matches the final PW injection interface and all required development scenarios are represented.

Validation

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