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).

CLAUDE.md

@@ -127,8 +127,8 @@ docs/  .gitea/workflows/  PLAN.md  CLAUDE.md  README.md
 gateway/  ui/  pkg/              # added by their stages
 platform/telegram/               # Telegram connector side-service (Stage 9): bot + gRPC API
 loadtest/                        # module scrabble/loadtest: the pre-release stress harness (R2)
-backend/Dockerfile gateway/Dockerfile platform/telegram/Dockerfile loadtest/Dockerfile  # multi-stage distroless (Stage 16; loadtest R2)
-deploy/                          # docker-compose + caddy + otelcol/prometheus/tempo/grafana (+ cAdvisor/postgres_exporter, R2)
+backend/Dockerfile gateway/Dockerfile platform/telegram/Dockerfile loadtest/Dockerfile  # multi-stage distroless (Stage 16; loadtest R2); gateway/Dockerfile also has the `landing` target (R3)
+deploy/                          # docker-compose + caddy + landing + otelcol/prometheus/tempo/grafana (+ cAdvisor/postgres_exporter, R2)
 ```
 
 ## Build & test
@@ -144,8 +144,9 @@ go run ./backend/cmd/backend    # /healthz, /readyz on :8080
 cd ui && pnpm install && pnpm check && pnpm test:unit && pnpm build  # the UI (Stage 7+)
 pnpm start                      # UI mock mode: lobby -> game, no backend
 
-docker build -f backend/Dockerfile -t scrabble-backend .   # images (Stage 16); gateway embeds the UI
-docker build -f gateway/Dockerfile -t scrabble-gateway .
+docker build -f backend/Dockerfile -t scrabble-backend .   # images (Stage 16); gateway embeds the SPA
+docker build -f gateway/Dockerfile --target gateway -t scrabble-gateway .
+docker build -f gateway/Dockerfile --target landing -t scrabble-landing .  # static landing (R3)
 docker compose -f deploy/docker-compose.yml config         # validate the full contour
 ```