snapshotChunkCeiling

Upper bound on the raw state bytes carried in a single §7 InstallSnapshot chunk. A fabric with a tighter framing limit shrinks chunks automatically: when the transport publishes a RaftTransport.maxPayloadBytes, the engine takes the lesser of this and what that budget leaves once the envelope reserve and CBOR's byte-array expansion are paid. The expansion matters — CBOR renders a ByteArray as an array of integers, so raw bytes cost up to two on the wire and a chunk sized against the budget directly would encode to roughly twice it (#2150).