Function resolveRuntime

The runtime a multi-input operation (concat, join, cross / as-of join) runs on:

1. an explicit runtime wins;
2. inputs on the process default runtime are unbound and adopt the others' runtime;
3. bound inputs must share one lineage (root), else RuntimeMismatchError — two tenants never mix implicitly;
4. within that lineage the receiver's runtime (its engine() fork) is kept when it belongs to it;
5. all inputs unbound: the receiver's runtime, else the first input's. The result never falls back to the process default when any input is bound.