Shared Types & Error Crate

[OlufunbiIK]

Complexity: Low

Problem: Type definitions and error codes are duplicated across contracts.

Scope: Build a shared crate with common types (AssetStatus, EscrowStatus, RiskGrade) and error enums used across all contracts.

Acceptance criteria: All contracts import from shared. No duplicate type definitions exist.

Files to create:
contracts/shared/src/types.rs
contracts/shared/src/errors.rs
contracts/shared/src/constants.rs