Add collateral_swap() replacing one collateral type with another without unstaking

[Rickyy1017]

Summary

Builds on Issue #214 (NFT collateral minting). Add a function that allows a staker with an active debt NFT to swap their underlying collateral token for a different token without unstaking — useful when the staker wants to change their exposure while keeping the loan active.

Acceptance Criteria

• CollateralSwapConfig stored per user: { debt_nft_id: u32, original_token: Address, swap_token: Address }
• initiate_collateral_swap(user, new_stake_token: Address, min_new_amount: i128) function
• Validates debt NFT exists and position is collateralized
• Unstakes current token, swaps via DEX to new_stake_token, re-stakes result
• min_new_amount is slippage protection — revert with SlippageExceeded if swap output below minimum
• Position lock maintained throughout — debt NFT remains valid against new token
• Debt NFT face_value updated to reflect new position amount
• collateral_swapped event: (user, old_token, new_token, old_amount, new_amount, ledger)
• Tests: swap executes correctly, slippage protection triggers, non-collateralized position rejected, face value updated correctly

Notes

• Depends on Issue Add staker_reputation_score() combining multiple engagement signals #214 (NFT collateral minting) for the collateralization mechanism.
• Uses DEX path payment — same pattern as Issue feat: position_value_in_reward_token, daily_reward_estimate, transfer_position_with_rewards, staking_efficiency_score (#132 #133 #134 #135) #147.