Type Alias CacheKeyExecutor

Cache-key-aware executor. When cacheKey is provided, the executor solves by reference (pointer: cacheKey) and falls back to a full upload on a server cache miss. Always reports the (possibly refreshed) cacheKey so the scheduler can update its definition→key map, plus whether the fast path missed (for telemetry). When cacheKey is null it's a first solve — upload fully and capture the key the server assigns.

Supplied by the client (which owns the solve primitives); the scheduler stays decoupled from the transport.