R3: dashboards, docs and tracker bake-back

- Edge/UX dashboard: aggregate request-rate vs rejection-rate panel
  (gateway_rate_limited_total by class; no per-user labels).
- ARCHITECTURE §2/§11/§12/§13: body cap + explicit h2c sizing, the rate-limit
  observability pipeline and auto-flag policy, the admin-limiter note (and the
  caddy-path gap), the landing container topology; fixed the stale 120/min
  per-user figure.
- FUNCTIONAL (+_ru): the Throttled view and the reversible high-rate flag.
- gateway/backend/deploy READMEs, TESTING.md, root CLAUDE.md updated.
- PRERELEASE.md: R3 interview decisions + implementation refinements logged;
  tracker R3 -> done (this PR implements it; CI gates the merge).

deploy/grafana/dashboards/edge-ux.json

@@ -34,6 +34,18 @@
       "fieldConfig": { "defaults": { "unit": "reqps" }, "overrides": [] },
       "datasource": { "type": "prometheus", "uid": "prometheus" },
       "targets": [{ "refId": "A", "expr": "sum(rate(edge_request_duration_count[5m])) by (result)", "legendFormat": "{{result}}" }]
+    },
+    {
+      "type": "timeseries",
+      "title": "Rate limiting — request rate vs rejections (R3)",
+      "description": "Aggregate only (no per-user labels, the Stage 12/17 discipline): total edge request rate against the limiter rejection rate by class. Per-key detail lives in the admin console's Throttled view.",
+      "gridPos": { "h": 8, "w": 24, "x": 0, "y": 16 },
+      "fieldConfig": { "defaults": { "unit": "reqps" }, "overrides": [] },
+      "datasource": { "type": "prometheus", "uid": "prometheus" },
+      "targets": [
+        { "refId": "A", "expr": "sum(rate(edge_request_duration_count[5m]))", "legendFormat": "requests" },
+        { "refId": "B", "expr": "sum(rate(gateway_rate_limited_total[5m])) by (class)", "legendFormat": "rejected · {{class}}" }
+      ]
     }
   ]
 }