Coffer single-vault per collateral type, not per position

Context

Two patterns were possible: one Coffer vault for all collateral, or per-instrument/per-position vaults.

Decision

One Coffer vault per supported collateral type (USDC for v1; USDT and ETH later).

Alternatives

Per-position vaults (rejected: defeats the cross-margin purpose). A single multi-asset vault (rejected: the ERC-4626 spec is single-asset).

Consequences

Simpler accounting, standard ERC-4626 compatibility, cross-margin is natural.