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".

deploy/.env.example

@@ -1,5 +1,5 @@
 # Environment for deploy/docker-compose.yml. The CI deploy job (ci.yaml) maps the
-# Gitea TEST_-prefixed secrets/variables onto these unprefixed names; Stage 17
+# Gitea TEST_-prefixed secrets/variables onto these unprefixed names; Stage 18
 # maps the PROD_-prefixed set the same way. Copy to deploy/.env for a local run.
 #
 # Full reference (required vs optional, defaults, secret-vs-variable): deploy/README.md.
@@ -17,7 +17,7 @@ LOG_LEVEL=info
 
 # --- Edge / caddy -----------------------------------------------------------
 # Test: ":80" (the host caddy terminates TLS and forwards to scrabble:80 on the
-# external `edge` network). Prod (Stage 17): a domain so caddy does its own ACME.
+# external `edge` network). Prod (Stage 18): a domain so caddy does its own ACME.
 CADDY_SITE_ADDRESS=:80
 GM_BASICAUTH_USER=gm
 GM_BASICAUTH_HASH=                     # required; `caddy hash-password` bcrypt hash