EngineBackend implementation backed by SQLite — dev-only per RFC-023.

Phase 1a scope

This crate is the RFC-023 Phase 1a scaffold: construction semantics (FF_DEV_MODE=1 guard, per-path registry, connection pool, WARN banner), the SQLite transient-busy error classifier skeleton, and an EngineBackend impl whose data-plane methods return EngineError::Unavailable. Phase 1b lands the hand-ported SQLite-dialect migrations; Phase 2+ replaces the Unavailable stubs with real bodies paralleling ff-backend-postgres.

Production guard

[SqliteBackend::new] refuses to construct without FF_DEV_MODE=1 in the process environment. The refusal is on the TYPE (RFC-023 §3.3 A3 / §4.5) so every path — embedded library consumers, ff_server::start_sqlite_branch, and ff_sdk::FlowFabricWorker::connect_with — pays the guard.

No ferriskey dep — this crate's transport is sqlx.