SDK: Define and export shared TypeScript types for contract data structures

[A6dulmalik]

Implementation path: sdk/src/types/index.ts (tests: sdk/src/types/__tests__/types.test.ts)

Define and export TypeScript types for all shared contract data structures used across SDK bindings.

Types to define:

• GroupInfo (Registry) — sdk/src/types/registry.ts
• SavingsGroup, Member, Contribution, Payout (Savings) — sdk/src/types/savings.ts
• GroupStatus, MemberStatus, Frequency enums (Savings)
• RegistryError, SavingsError error enums

These types should be the single source of truth imported by all individual binding modules.
Includes unit tests asserting type shapes match contract ABI.