Refine typed entitlement readiness

[lynnswap]

Summary

• derive activeEntitlements as an optional typed projection of the verified entitlement snapshot
• distinguish unresolved startup state (nil) from a resolved account with no matching entitlement (an empty set)
• avoid a second stored observable entitlement state
• update the README, DocC overview, and external consumer example for the readiness contract
• add coverage for unresolved, empty, populated, and startup-recovery projections

Why

The previous non-optional empty set could make consumers treat an entitlement query that had not completed as a confirmed account with no entitlement. Making the typed projection optional preserves StoreKit readiness semantics and keeps entitlements as the single source of truth.

Validation

• swift test --build-system swiftbuild --no-parallel — 27 tests in 7 suites passed on macOS
• xcodebuild test -scheme StoreTransactionKit -destination 'platform=iOS Simulator,id=C2892BF2-B747-45AF-ABD6-0CF9DFC1374A' — 27 tests in 7 suites passed
• swift build --build-system swiftbuild in Fixtures/ExternalConsumer
• xcrun swift-format lint --recursive --strict Sources Tests Package.swift
• ruby scripts/verify-github-actions-pinning.rb
• Codex review completed with no actionable findings