Borrow and Repay - Vanna

`user_open_debt_position` / `user_close_debt_position`

Open once per (margin, reserve). Close when shares are zero.

`user_borrow(assets, max_debt_shares)`

Accrue. Transfer cash to the margin vault. Mint debt shares ≤ max_debt_shares. Full-set health > 1.1.

`user_repay_from_margin(max_assets, repay_all)`

Burn shares from vault cash.

`public_repay_from_wallet(max_assets, repay_all)`

Anyone pays. Useful when the vault is stuck in another asset.

`user_deposit_and_borrow(deposit_amount, borrow_amount, max_debt_shares)`

Atomic composite. Perps open uses this plus swap in the same transaction on the client.