Module engine_error 

Typed engine-error surface (issue #58.6).

RFC-012 Stage 1a: moved from ff-sdk::engine_error to ff-core::engine_error so it becomes nameable by the EngineBackend trait (which lives in ff-core::engine_backend) without forcing a public-surface dependency from ff-core on ff-script. The [ScriptError]-aware helpers (From<ScriptError>, valkey_kind, transport_script, transport_script_ref) live in ff-script as free functions (see ff_script::engine_error_ext) — ff-core owns the enum shapes; ff-script owns the transport-downcast plumbing.

Mapping shape

ScriptError lives in the ff-script crate (transport-adjacent). EngineError lives here in ff-core and is what public SDK calls return via ff_sdk::SdkError::Engine. The bidirectional mapping:

• From<ScriptError> for EngineError — every ScriptError variant is classified into NotFound / Validation / Contention / Conflict / State / Bug / Transport. Parse + Valkey flow through Transport { source: Box<ScriptError> } so the underlying ferriskey::ErrorKind / parse detail is preserved.
• DependencyAlreadyExists is special: per the #58.6 design the variant carries the pre-existing [EdgeSnapshot] inline. Populating that field requires an extra round-trip (the Lua script only knows the edge_id), so plain From<ScriptError> returns a Transport fallback for that code — callers in the stage_dependency path use ff_sdk::engine_error::enrich_dependency_conflict to perform the follow-up describe_edge and upgrade the error before returning.

Exhaustiveness

The top-level EngineError and every sub-kind are #[non_exhaustive]. FF can add new Lua error codes in minors without a breaking change to this surface — consumers that match on a sub-kind must include a _ arm.

Enums

BugKind

FF-internal invariant-violation sub-kinds. Should not be reachable in a correctly-behaving deployment.

ConflictKind

Permanent conflict sub-kinds. Caller must reconcile rather than retry.

ContentionKind

Contention sub-kinds (retryable per RFC-010 §10.7). Caller should re-dispatch or re-read and retry.

EngineError

Typed engine-error surface. See module docs.

StateKind

Legal-but-surprising state sub-kinds. Per-variant semantics vary (some are benign no-ops, some are terminal). Consult the RFC-010 §10.7 classification table.

ValidationKind

Validation sub-kinds. 1:1 with the Lua validation codes.