Dev-client hides derived-field decode rejection behind error-free JSON and shell exit1 #578

Original retained dev-client shell exits1 after eight successful public replies, without visible JSON error.

Determined offline against exact qualified8513 target and original closed journal: registration-status rejects declared derived CustomFunctionRegistrationObservation.outcome because original python_introspect.dataclass_from_mapping admits only init=True fields, while canonical serializer emits all fields. McpDevPayloadFailure carries mcp_payload_invalid, but its JSON projector emits only the raw receipt and hides that diagnostic. Shell's nonzero aggregation is therefore justified by an invisible local decode failure, not a native operation failure.

Singer owns OpenHCS result/diagnostic family; declaration decoder seam coordinated with Dewey. No current open dev-client owner or Root394 crossing found. No new client, registration replay, science input, timeout/cap increase or environment change. Original shell terminal1 and merged567 native positive remain retained.

Acceptance: original successful saved reply round-trips through its declared owner, while supplied contradictory derived state/unknown fields fail closed. Any genuine decode rejection remains visible through JSON and formatting with original receipt retained and nonzero returncode. Independent derived declaration needs no generic consumer switch. Original shell/user entrypoint verification after coherent fix; no UNKNOWN request replay.