Stage 16: insert Stage 17 (test-contour verification); renumber prod deploy to 18

- PLAN.md: new Stage 17 "Test-contour verification & defect fixes" (exercise the
  deployed contour end-to-end and fix what it surfaces — connector liveness check,
  path-conditional CI); the former prod-deploy stage becomes Stage 18.
- Renumber every "Stage 17" prod-deploy reference to "Stage 18" across docs,
  compose, Caddyfile, ci.yaml and CLAUDE.md; the post-Stage-14 split range is now
  "Stages 15–18".

README.md

@@ -98,6 +98,6 @@ docker compose -f deploy/docker-compose.yml config         # validate (needs the
 
 CI auto-deploys the **test contour** on a PR into — or push to — `development`
 (`.gitea/workflows/ci.yaml`); the **prod contour** is a manual deploy after
-`development → master` (Stage 17). Env reference: [`deploy/.env.example`](deploy/.env.example);
+`development → master` (Stage 18). Env reference: [`deploy/.env.example`](deploy/.env.example);
 the topology and the two-contour model are in
 [`docs/ARCHITECTURE.md`](docs/ARCHITECTURE.md) §13.