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/docker-compose.yml

@@ -11,7 +11,7 @@
 #   - `edge` (external): the host caddy reaches this contour at `scrabble:80`
 #     (the in-compose caddy's alias). The in-compose caddy terminates only HTTP in
 #     the test contour; the host caddy terminates TLS and forwards. For prod
-#     (Stage 17, no host caddy) set CADDY_SITE_ADDRESS to the domain so the caddy
+#     (Stage 18, no host caddy) set CADDY_SITE_ADDRESS to the domain so the caddy
 #     does its own ACME — the contour is then self-contained.
 #   - The connector egresses to api.telegram.org through the `vpn` sidecar
 #     (network_mode: service:vpn); it answers internal gRPC at `telegram:9091`.