Async Button surface, LocalizableError, and alert(error:)

[foscomputerservices]

Summary

Adds the async action surface designed and ratified in planning/stream/feat-async-button-surface.md:

• Async Button forms — every Localizable-titled Button form (and the ViewBuilder forms) gains an async twin: the action is @Sendable () async throws, a thrown error lands in a required error: binding (cleared on each launch — the binding holds the outcome of the most recent invocation). An optional caller-owned AsyncButtonActivity adds deterministic re-entry refusal plus running-state for disabled(_:)/progress. Providing the cancel face (cancelTitle:/cancelSystemImage:/cancelImage: or a phase-aware label) makes the button two-faced — tap to start, tap again to cancel — with a cancelling phase while the work unwinds cooperatively and a refractory window against taps aimed at a face that just flipped. All tap semantics live once, in AsyncButtonEngine; the 12 titled forms are generated by the overload sweep's new Stage 6b, cross-checked against Button's action-taking Localizable surface on every full sweep (--emit-async-only re-renders just that file from the checked-in SDK stamp).
• LocalizableError + @LocalizableError — errors compose like ViewModels: @LocalizedString/@LocalizedSubs message, plumbing from the macro, exposed as localizedMessage. Localization is the localizing encode — ErrorMiddleware resolves the message on the way out, so wire errors arrive resolved. options: [.clientHosted] marks client-created errors (ClientHostedLocalizableError); localized(mvvmEnv:locale:) resolves them at presentation via the same round-trip ClientHostedViewModelFactory runs for a ViewModel. One localization domain per error type.
• alert(error:title:message:dismissButtonLabel:) — one screen-level presentation point for the shared error binding, implemented as pure Localizable twin composition; the %{error} substitution point is filled with a typed Localizable.
• Docs & skills — DocC article Async Actions and Error Presentation, api-catalog entries + reach-for index, CHANGELOG under [Unreleased]; generator skills now teach this shipped API (replacing stale helper shapes that had crept in).

Testing

• 43 new tests across five suites: engine semantics (one per ratified behavior, incl. refractory and cancellation), generated-surface construction, wire + client-hosted LocalizableError flows (en/es), alert substitution ladder, and macro expansion.
• Full swift test green; api-catalog audit clean (0 gaps, 0 stale); swiftformat fixed-point and swiftlint clean on all changed files.
• CI to verify what a 27.0-beta-SDK machine cannot: the sweep --check leg (first run of verifyAsyncTwinBase + byte-compare of Button+AsyncAction.swift on 26.5 SDKs), Linux legs, and per-platform xcodebuild legs.