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".

docs/ARCHITECTURE.md

@@ -569,7 +569,7 @@ Two contours, two secret/variable prefixes (`TEST_` / `PROD_`):
   host, then a `GET /` probe through caddy). The host caddy terminates TLS and
   forwards the domain to `scrabble:80`, so the in-compose caddy serves plain HTTP
   (`CADDY_SITE_ADDRESS=:80`).
-- **Prod** (Stage 17): a manual SSH deploy after `development → master`. There is no
+- **Prod** (Stage 18): a manual SSH deploy after `development → master`. There is no
   host caddy, so the contour ships its own caddy terminating TLS — set
   `CADDY_SITE_ADDRESS` to the domain and the caddy does its own ACME.