lookupBackend method
Phase-0 unification scaffold: look up a table by name across both
the in-memory and paged backends and return the shared
TableBackend view of it. Returns null when no table by that
name exists in either registry. Prefer this over poking at
_tables / _pagedTables directly when you only need metadata
(existence / column names / which backend it lives on).
Implementation
TableBackend? lookupBackend(String name) {
final t = _tables[name];
if (t != null) return t;
return _pagedTables[name];
}