Add deterministic access-loss testing and API polish

[lynnswap]

Purpose

Complete the remaining design-audit work around subscription access testing and consumer-facing API clarity.

Changes

• add expireActiveSubscription() with ready-empty publication, admission checks, and replay-safe expiration state
• add delivery-only synthetic transactions for testing arbitrary StoreKit classification and delegate paths through the production pipeline
• clarify exact tier identity versus app-owned feature inclusion in README and DocC
• make StoreTransactionError conform to LocalizedError
• document why transaction history is all-or-nothing
• extend internal and external-consumer contract coverage for access loss, hierarchy, lifecycle errors, unmanaged transactions, and catalog contradictions

Testing

• swift test — 134 tests passed
• swift test --package-path Fixtures/ExternalConsumer — 4 tests passed
• xcrun swift-format lint --recursive --strict Sources Tests Tools/TestApp Package.swift
• xcrun swift-format lint --recursive --strict Fixtures/ExternalConsumer/Sources Fixtures/ExternalConsumer/Tests Fixtures/ExternalConsumer/Package.swift
• xcodebuild docbuild -scheme StoreTransactionKit -destination 'generic/platform=macOS'
• branch-wide Codex review against main — no findings