feat: implement token holder lock/unlock with transfer gate

[7maylord]

Summary
Token holders needed to be temporarily non-transferable during events like voting windows or vesting periods. The transfer gate and storage key (TokenLockedUntil) were already in place, but lock_tokens had no authorization enforcement and is_tokens_locked did not exist at all, leaving the feature incomplete and insecure.

• is_tokens_locked — a missing read-only check to query whether a holder's tokens are currently locked

• Add caller: Address parameter to lock_tokens and enforce that only the asset's tokenizer can lock a holder's tokens (previously unauthenticated — any caller could lock anyone's tokens)

• Fixed existing test_lock_tokens call to pass the new caller argument

• Added setup_tokenized helper to avoid repeating the tokenize boilerplate across the new tests

• Fixed Compatibility Bugs by refactoring the entire AssetsUp Soroban smart contract codebase to be compatible with Soroban SDK v22. On the source side, this involved migrating all numeric types from BigInt to native i128 across types.rs, tokenization.rs, dividends.rs, voting.rs, detokenization.rs, and lib.rs; removing the custom ContractEvent struct and replacing all event emissions with inline tuples passed directly to env.events().publish(); restructuring DetokenizationProposal from a flat enum to wrapper structs (ActiveProposal, ExecutedProposal, RejectedProposal) each annotated with #[contracttype]; and fixing all persistent storage reads from the old double-ok_or pattern (store.get().ok_or(E)?.ok_or(E)?) and Some(Some(v)) match arms to the SDK v22 single-Option

closes #469

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