[NF-16] [CONTRACT] Batch Multi-Tier Deposit

Overview

A user who wants to spread capital across multiple tiers in one sitting currently has to submit one transaction per tier. This issue adds a single-transaction batch deposit.

Problem

VaultRouter.deposit(user, tier, amount) accepts exactly one tier/amount pair per call. Building a ladder across Flex/L3/L6/L12 in a single sitting today costs 4 separate transactions, 4 separate USDC transfers, and 4 separate signature prompts in the wallet.

Proposed Solution

• Add VaultRouter.batch_deposit(user: Address, deposits: Vec<(Tier, i128)>).
• Validates every (tier, amount) pair up front (all tiers valid, all amounts meet their tier minimum) before moving any funds, so the whole batch is atomic — either every deposit in the batch succeeds or the entire transaction reverts, with no partial state where some tiers received funds and others didn't.
• Internally loops the existing single-tier deposit logic per pair, reusing that code path rather than duplicating it.
• Single USDC transfer for the summed total where possible, or per-tier transfers if that's simpler to keep the existing per-vault forwarding logic intact — implementation detail for the PR, but must not double-charge or under-charge the depositor relative to the sum of individual deposits.

Acceptance Criteria

• A single batch_deposit call correctly creates positions in all specified tiers
• If any single pair in the batch is invalid (bad tier or below minimum), the entire transaction reverts — no partial deposits
• Total USDC moved equals the exact sum of the batch's amounts, no more, no less
• Gas/resource cost of one batch_deposit for N tiers is meaningfully lower than N separate deposit calls
• Unit tests cover: successful 4-tier batch, one invalid pair causing full revert, batch with duplicate tiers (define and test the intended behavior — reject duplicates or sum them, documented explicitly in the PR)

Start at the existing VaultRouter.deposit entry point and trace its validation, transfer, and per-vault forwarding behavior before defining the batch path. Review the contract's unit-test setup, then cover the four-tier success case, atomic failure, exact total transfer, gas/resource improvement, and an explicitly documented duplicate-tier behavior.