Add get_tvl view function to Vault

[Code-Paragon]

Problem

External protocols and the frontend analytics module have no direct, on-chain way to query the Total Value Locked (TVL) without manually checking the underlying token balance of the contract address.

Expected behavior

A get_tvl read-only function is implemented that queries the underlying token client and returns the exact base asset balance held by the Vault.

Files to update

• src/vault.rs
• src/lib.rs

Project relevance

This establishes a standard API for composability, allowing other DeFi primitives and the SDK's AnalyticsModule to securely fetch the Vault's liquidity depth.

Acceptance criteria

• Feature works as described
• Tests are added or updated where needed
• Documentation is updated where needed
• Function correctly instantiates a token::Client and returns the contract's balance
• No TypeScript, lint, test, or build errors