op-devstack: support externally implemented L2 consensus clients

op-devstack currently constructs stock L2 consensus clients internally, and several exported presets do not expose enough launch context to substitute an alternate implementation without maintaining a downstream fork. This also prevents callers from preserving no-supernode multi-chain and deliberately staged startup semantics.

Expose a product-neutral external L2 CL slot contract, a two-L2 interop runtime that does not insert a stock supernode into the correctness path, and a staged variant that lets callers attach CLs after their execution engines exist. A nil factory must preserve existing behavior, while unsupported partial fallback combinations must fail closed.

The implementation will be delivered as focused stacked PRs.

Start by tracing op-devstack's stock L2 consensus-client construction and the exported presets that lack launch context. Define the external L2 CL slot contract, then verify the two-L2 interop and staged variants preserve no-supernode and startup semantics. Done means nil factories retain existing behavior and unsupported partial fallback combinations fail closed.

Written by the indexing model from the issue text.