FR: Resource-Timing-style per-request timing breakdown

[passcod]

🤖 Browsers expose per-request phase timings (DNS / connect / TLS / TTFB / download) via Resource Timing. For a server-side client this is observability gold, and fáith already has the stats/conn-tracker scaffolding to hang it on.

The constraint is what reqwest exposes: total time, connection reuse, and remote address are easy; phase boundaries (DNS vs connect vs TLS) are not, without upstream hooks. Even the partial version is most of the practical value:

response.timing = {
  start,        // request accepted
  sent,         // request handed to the connection
  firstByte,    // response headers received
  end,          // body fully consumed (settled late, like trailers)
  reusedConnection: bool,
  protocol: "HTTP/2.0",
}

Middleware-level timestamps get start/firstByte; body-stream completion gets end. Worth designing the shape against the PerformanceResourceTiming names where they map, so it feels familiar.

The per-protocol path-time EWMA from #45's follow-on records origin-level aggregates of the same measurements — the two should share the instrumentation point.