fix: expose monthly equivalent display text

[Kapusch]

Summary

• expose optional MonthlyEquivalentPriceDisplayText at the end of the public offer metadata contract
• derive it only for StoreKit subscription periods of exactly one year
• divide Product.price by 12 and format the result with product.priceFormatStyle
• document the optional metadata

Validation

• native Swift wrapper built for device arm64 and simulator arm64 and x86_64
• package project Release build passed
• sample iOS Release build passed
• NuGet 1.0.0-rc.2 packed and package layout validated
• workflow YAML parsed successfully

Release sequence

After review, merge into master, require CI to pass, then publish tag v1.0.0-rc.2 from master according to Docs/Release.md.