Add repeat/separator templates; validate at compile time; support Go 1.22+ via GO_VERSION

compose.yaml

@@ -8,9 +8,12 @@
 # Build caches persist in the `gocache` volume, so re-runs are fast and the
 # host tree stays clean. Commands that rewrite source (fmt, tidy) keep your
 # ownership when run with `--user "$(id -u):$(id -g)"`.
+#
+# GO_VERSION picks the toolchain image (default: latest stable). Test the lowest
+# supported Go with: GO_VERSION=1.22 docker compose run --rm test
 
 x-go: &go
-  image: golang:1.26.4
+  image: golang:${GO_VERSION:-1.26.4}
   working_dir: /app
   volumes:
     - .:/app