[Smart Contract] Implement Soroban Identity Contract

[mftee]

Description:
Create the on-chain identity registry contract. This stores a mapping from a Stellar wallet address to a hashed user ID (BytesN<32>), enabling on-chain proof that a wallet belongs to a registered FreightFlow user.

Create:

• contracts/identity/Cargo.toml
• contracts/identity/src/lib.rs

Acceptance Criteria:

• Cargo.toml: crate-type = ["cdylib"], soroban-sdk = { version = "22.0.0", features = ["alloc"] }, testutils feature gate, release profile with lto = true, panic = "abort"
• IdentityError enum with #[contracterror]: AlreadyRegistered = 1, NotRegistered = 2, Unauthorized = 3, NotInitialized = 4
• DataKey enum: Identity(Address), Admin
• initialize(env, admin: Address) — one-time setup; returns Err(NotInitialized) if re-called
• register_identity(env, user_id_hash: BytesN<32>, wallet: Address) — wallet.require_auth(), store mapping, extend TTL ~1 year (6,307,200 ledgers)
• verify_identity(env, wallet: Address) → bool
• get_user_identity(env, wallet: Address) → Result<BytesN<32>, IdentityError>
• revoke_identity(env, wallet: Address) — admin-only
• 4 unit tests (using Env::default() + env.mock_all_auths()): register+verify success, double-register returns AlreadyRegistered, revoke removes entry, get on unregistered returns NotRegistered
• Added to contracts/Cargo.toml workspace members