SCC engine facade: one orchestration seam for every transport.
The scc CLI is one client of this engine — never the only way to
access functionality. CLI, HTTP, MCP, SDKs, RPC, and FFI all call
these operations; business logic lives here, transports only parse
arguments and render results.
Migration rule (spec §50): installing no third-party plugins preserves existing behavior. This facade moves orchestration WITHOUT changing algorithms — outputs stay byte-identical (parity tests enforce it).