Display stream metrics (TTFT, tokens, latency) after each turn

[rdwj]

The fipsagents SSE serializer already emits a `stream_metrics` chunk with TTFT, total time, token counts, and inter-token latencies. The UI just needs to consume it and render a small metrics bar under each assistant message.

Reference: tool-calling-demo
See UPSTREAM_PORT.md sections 1.3, 1.8.