The SSE streaming path hardcoded '耗时: -' because duration_ms was
never threaded through:
- docker.rs: OutputChunk::Done now carries duration_ms, computed from
start_container to wait completion via Instant::elapsed
- sse.rs: DonePayload gains duration_ms, serialized into the done event
- runner.rs: WASM done callback formats the real duration instead of '-'
The polling fallback path already had correct duration via ExecResult;
only the SSE path was missing it.