[Feature] V2: implement credit accrual and owner-authorized withdrawal

[collinsezedike]

Summary

Implement the withdrawal step that turns settled Credit(id, address) entitlements (see #69) into actual token transfers, and its reentrancy guard.

What to build

withdraw(owner: Address, id: u64, destination: Address)

• Requires owner's authorization.
• Reduces Credit(id, owner) and the dispute's outstanding liability, and increases its withdrawn total, before making the outgoing token transfer (effects before interactions).
• If the token transfer fails, atomically roll back all three of the above; the credit is not consumed by a failed transfer.
• The owner may specify any destination address, so a token that rejects transfers to owner directly can't permanently strand funds, the owner can route around it.

Reentrancy guard on incoming deposits (this affects registration in #66 as well, call it out there too if not already handled): a position must not become usable, by a permissionless cutoff transition or by a settlement callback, while its funding token transfer is still executing. Enter the guard before the external call; roll it back with the transaction on transfer failure. A position only becomes active as part of a successfully completed funding operation.

Invariant to test explicitly: total withdrawals and credits for one dispute must never exceed its funded positions, even under a hostile token contract that tries to call back into Tholos mid-transfer.

Depends on

#69 (credit must be accrued by settlement before it can be withdrawn).