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/caddy/Caddyfile

@@ -4,7 +4,7 @@
 # Connect edge) goes to the gateway. Mirrors ../galaxy-game's /_gm model.
 #
 # CADDY_SITE_ADDRESS is ":80" in the test contour (the host caddy terminates TLS
-# and forwards); set it to a domain in prod (Stage 17) so this caddy does its own
+# and forwards); set it to a domain in prod (Stage 18) so this caddy does its own
 # ACME and the contour is self-contained.
 {
 	admin off