Expose subscription catalog metadata inputs

[lynnswap]

Purpose

Expose the validated subscription catalog as the typed bridge to StoreKit product metadata while keeping transaction snapshots focused on verified transactions.

Changes

• expose the subscription group ID and declaration-ordered StoreKit product IDs
• add exact product ID-to-entitlement lookup while preserving optional entitlement values
• update Quick Start and DocC with Product and Product.SubscriptionInfo integration
• verify the public contract from package and external consumer tests

Testing

• xcrun swift-format lint --recursive --strict Sources Tests Tools/TestApp Package.swift
• swift test --build-system swiftbuild --no-parallel
• swift build --build-system swiftbuild --configuration release
• external consumer swift test --build-system swiftbuild
• Xcode builds for iOS, Mac Catalyst, tvOS, watchOS, and visionOS
• xcodebuild docbuild for macOS
• Codex Review against main with no findings