feat(google): loyalty balance and account name on store cards

[alexpate]

What

Adds google.loyalty to store cards so Google Wallet loyalty objects can show a native balance and the holder's name:

• points / secondaryPoints: { label, balance } become loyaltyPoints / secondaryLoyaltyPoints. A string balance is sent as balance.string (so "6 / 10" works as-is), an int32 as balance.int, any other finite number as balance.double.
• accountName: the holder's name. null leaves it off the object.
• New exported types: GoogleLoyalty, GoogleLoyaltyPoints. README section and a minor changeset included.

Why

Store cards always put the pass description in accountName, which Google defines as the holder's name (for example "John Smith"). Stamp cards therefore showed "Stamp card for ..." there and had no native balance.

Testing

• Render tests: each balance type, int32 overflow to double, secondary points, accountName set and null, class output unchanged.
• Schema tests: accepted shape, rejection of empty label, empty string balance, NaN balance and empty accountName; other styles still strip google.
• Full gate green: lint, type-check, build, test, bundle-guard, size (13.34 kB of 15 kB), test:workers, publint, attw.

Compatibility

Opt-in. Without google.loyalty the output is byte-for-byte what 0.6.0 renders, and accountName still defaults to the description (pinned by a full-payload test whose expected output was captured from the 0.6.0 release). Both fields are object-level, so a balance change never re-submits the class for review. Other styles ignore the key (it's stripped, as unknown keys always were). A malformed google value on a store card is now a schema error (it used to be stripped silently).