server/table_backend library

Phase 0 unification scaffold.

TableBackend is the minimal shared surface between the in-memory Table and the out-of-core PagedTable. Today the SQL executor in Database still branches on the concrete backend type, but every new caller should prefer this interface so the per-site migration to a uniform dispatch path can be done incrementally.

The interface is intentionally tiny — it exposes only the metadata needed for "does table X exist?" / "what columns does it have?" / "which backend is it?" queries. The real divergence in row I/O (sync iteration vs async streams) stays on the concrete classes until the executor is ready to be unified.