[GF-03] Implement Locked Vault Trio (VaultL3, VaultL6, VaultL12)

[SharifIbrahimDev]

Description

This PR implements the three time-locked vault contracts (VaultL3, VaultL6, VaultL12) to fulfill issue #36.

Since the contracts/shared crate from GF-01 (#34) has not been merged yet, I embedded the required fixed-point math (mul_fp) and the Checkpoint storage key logic internally inside each vault as a mock setup to avoid blocking the implementation. These can be refactored to use the shared crate once GF-01 is merged.

Key Changes

• VaultL3: 3-month lock (777,600 ledgers), 1.05x multiplier, 0.50% exit fee, 50 USDC min deposit.
• VaultL6: 6-month lock (1,555,200 ledgers), 1.15x multiplier, 1.25% exit fee, 100 USDC min deposit.
• VaultL12: 12-month lock (3,110,400 ledgers), 1.40x multiplier, 3.00% exit fee, 250 USDC min deposit.
• Deposit Flow: Validates min deposit, scales shares using mul_fp against the tier multiplier, sets LockUntil, sets the deposit_ledger + 1 checkpoint, and forwards capital to StrategyVault.
• Withdraw Flow: Asserts the lock duration has expired, burns shares, and cleans up the persistent storage keys.
• Early Exit Flow: Calculates the tier-specific exit fee using mul_fp, deducts it from the principal, and socialises the fee by leaving the capital inside the StrategyVault to increase the pro-rata value of all remaining shares.
• Tests: Comprehensive test coverage including tests for deposit logic, lock panics, successful withdrawals at maturity, and early exit fee calculations.

Closes #36

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