test: measure client-observed refresh latency (Refs #531)

Separate round-trip and request-relative TTFE metrics from discovery and startup timing. Preserve exact-base legacy comparisons during the schema-v3 rollout and require the new distributions without mislabeling old samples.

Exercise the real client request path with deterministic transport timing, reject malformed environment notifications before recording samples, and document sequential attribution and remaining hosted calibration.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: karthiknadig