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

[mftee]

Changes

• Add PositionCount and Position storage keys to vault_l3, vault_l6, vault_l12
• Create Position struct with principal, shares, lock_until, checkpoint
• Modify deposit to create new position instead of merging (returns position_id)
• Add position_id parameter to withdraw and early_exit
• Add positions(user) function to return all user positions
• Keep balance(user) and shares(user) as aggregate sums for backward compatibility
• Update VaultRouter to pass position_id through

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 open positions with correct principal/shares/lock_until per entry
• balance(user)/shares(user) aggregate correctly across all open positions

closes #76

[vercel]

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