R7: trip report + docs/tracker bake-back; mark R7 done

- loadtest/REPORT-R7.md: the final stress-run report — method, the 500-player resource
  profile, the agreed tuning, the validation (transport_error 2.49% -> 0.72% at 3 gateway
  cores; the burst run showing connection-bound behavior), and the prod-sizing
  recommendation for Stage 18.
- loadtest/README.md: per-player transports, --cpus capping, docker_stats (was cAdvisor),
  the absolute BACKEND_DICT_DIR for ./loadtest/... , and report links.
- docs/TESTING.md + docs/ARCHITECTURE.md: observability now uses the otelcol docker_stats
  receiver (cAdvisor removed); links to both trip reports.
- CLAUDE.md: repo-layout line reflects docker_stats + per-service limits.
- PRERELEASE.md: R7 marked done in the tracker + heading; a Refinements entry recording
  the decisions, findings, applied tuning and validation.

This is the final pre-release hardening phase; Stage 18 (prod cutover) is next.

docs/TESTING.md

@@ -127,8 +127,10 @@ tests or touching CI.
   selection, the report); the DAWG-backed move test runs under `BACKEND_DICT_DIR` (as the
   engine tests do). It is **not** part of the per-PR suite's behavioural assertions: it
   runs ad hoc as a one-shot container against the contour, producing a trip report (bugs
-  + a resource baseline) read off the **cAdvisor + postgres_exporter** Grafana dashboard
-  on the contour. See [`../loadtest/README.md`](../loadtest/README.md).
+  + a per-container resource profile) read off the **otelcol `docker_stats` +
+  postgres_exporter** Grafana dashboard on the contour. Two passes are recorded — the
+  early [`REPORT-R2.md`](../loadtest/REPORT-R2.md) and the final, tuned
+  [`REPORT-R7.md`](../loadtest/REPORT-R7.md). See [`../loadtest/README.md`](../loadtest/README.md).
 
 ## Principles