EngineError gains Cancelled and ModelMissing { model: AiModel } and a
kind() -> ErrorKind, so frontends decide on what an error is rather than
on its words. The face detector, SAM 3 model checks and the Lightroom
job produce them at the source; the error text is unchanged, so control
channel and MCP clients see the same `error` strings.
Background results that cross the wire as JSON get an additive flag
instead: SAM 3 download status `cancelled`, Apple Photos import outcome
`cancelled: true` next to `skipped`.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
