Add SDK vault operation preview builder

[El-swaggerito]

Description

Difficulty: Expert

Type: Feature

Summary

Add a vault operation preview builder for deposit, withdraw, lock, and balance-related actions.

Current Behaviour

Vault operations may not have a consistent preview object for applications to display before user action.

Expected Behaviour

The SDK should provide a typed preview model showing operation type, amount, asset, wallet, network, estimated fee, and warnings.

Suggested Implementation

Create preview types, builder utilities, validation, fixtures, tests, and docs.

Files or Areas Likely Affected

• src/vault/
• src/transactions/
• src/types/
• src/errors/
• tests/
• docs/

Acceptance Criteria

• Vault operation preview builder is implemented.
• Deposit, withdraw, lock, and balance-related previews are supported.
• Invalid preview input returns typed errors.
• Warnings are included where useful.
• Tests cover major preview states.
• Docs explain preview usage.

Additional Notes

This should support safe review-before-action UX.