[NF-01] [CONTRACT] Multi-Position Support Per Tier Vault (True Laddering) #76

Overview

The protocol is named YieldLadder, but no vault currently lets a single user hold multiple independent positions in the same tier with staggered maturities — the core idea of a bond/CD ladder. This issue adds real multi-position support to vault_l3, vault_l6, and vault_l12.

Problem

Each locked tier vault stores exactly one balance record per user: DataKey::Balance(Address), DataKey::Shares(Address), DataKey::LockUntil(Address). In the current deposit():

let current_balance = ...Balance(user.clone())... .unwrap_or(0);
env.storage().persistent().set(&DataKey::Balance(user.clone()), &(current_balance + amount));
...
let lock_until = env.ledger().sequence() + lock_duration;
env.storage().persistent().set(&DataKey::LockUntil(user.clone()), &lock_until);

A second deposit into the same tier merges principal/shares into the same slot and overwrites LockUntil with a fresh now + lock_duration for the entire combined balance — the first deposit's original maturity date is lost. Users cannot actually build a ladder of staggered maturities within one tier today.

Proposed Solution

• Add DataKey::PositionCount(Address) and DataKey::Position(Address, u32) (a struct with principal, shares, lock_until, checkpoint) to each locked tier vault.
• deposit(user, amount) -> u32 creates a new position record and returns its position_id, instead of merging into a single slot.
• withdraw(user, position_id) -> i128 and early_exit(user, position_id) -> i128 operate on one position at a time.
• Add positions(user) -> Vec<Position> and keep balance(user)/shares(user) as aggregate sums across all of a user's positions for backward compatibility with existing callers (VaultRouter).
• Update VaultRouter's deposit/withdraw/early_exit to pass through position_id where applicable.

Acceptance Criteria

• A user can hold 2+ concurrent positions in the same tier vault with independent lock_until values
• A second deposit does not alter the maturity of any prior position
• withdraw/early_exit require a position_id and only affect that position
• positions(user) returns all of a user's open positions in the tier with correct principal/shares/lock_until per entry
• balance(user)/shares(user) aggregate correctly across all open positions
• Unit tests cover: two deposits with different lock_until, independent withdrawal of one position while the other remains locked, early_exit of one position not affecting others